Read the passage then choose the best answer to each question that follows. Identify your answer by writing the corresponding letter A, B, C or D on your answer sheet.

THE ATMOSPHERE OF VENUS

Venus, also called the Morning Star and Evening Star, is the second-closest planet to the sun and the brightest object in the night sky. The planet orbits the sun every two hundred and twenty four Earth-days and is sometimes referred to as Earth’s sister planet because the two share both a similar size and bulk. What is not similar, however, is Venus’s atmosphere in comparison to Earth’s atmosphere.

The atmosphere on Venus is much heavier and has a higher density than that of Earth. Venus’s atmosphere also expands significantly higher than Earth’s atmosphere although a thick cloud cover makes the surface of Venus nearly impossible to see unless observed through radar mapping.

While the pressure and temperature of Venus’s upper atmosphere are comparable to those of Earth, the heat and pressure of the lower atmosphere are not unlike a furnace. Venus’s atmosphere is very thick due to a composition consisting mainly of carbon dioxide, and a small amount of nitrogen. If man could survive the extreme heat of Venus’s surface (400 degrees Celsius), then he would have to contend with a surface pressure that is more than 90 times that of Earth. Venus’s extremely high temperature is thanks to the greenhouse effect caused by such a large amount of carbon dioxide. The greenhouse effect is a process by which the sun’s infrared radiation is more readily absorbed by the atmosphere. Just like in a real greenhouse used to grow plants years round, the proliferation of carbon dioxide traps radiation and warms Venus’s atmosphere. Due to this phenomenon, Venus boasts a higher atmospheric temperature than Mercury, even though Venus is twice the distance from the sun.

However, scientists postulate that Venus’s atmosphere was not always so hot. [A] Studies show that large bodies of water were once on Venus’s surface but that eventually evaporation of all the water caused the runaway greenhouse effect which regulates the planet today. [B] Thus Venus has become a critical study for today’s scientists, as human being are only beginning to struggle with the early stages of the greenhouse effect. [C] Our problems do not stem from evaporated water supplies but from a propagation of carbon dioxide and other greenhouse gases due to industrial and automobile emissions. [D]

Another interesting characteristic to note regarding Venus’s atmosphere is that its daytime temperatures and nighttime temperatures are not that far removed from each other. This is due to the thermal inertia, the ability of a substance to store heat despite changing temperatures and the transfer of heat by Venus’s strong winds. Although winds on the surface of Venus move slowly in comparison with Earth’s winds, Venus’s air is so dense that a slow-moving there can move large obstructions and even skip stones along the planet’s surface.

In 1966, humankind made its first attempt at sending a recording instrument into Venus’s atmosphere. The Venera 3 probe did collide with Venus surface; however, the abrupt impact caused its communication system to fail, and it was unable to send and feedback. In 1967, Venera 4 successfully enter Venus’s atmosphere and was able to take many readings, one of which recorded that Venus’s atmosphere was between ninety and ninety-five percent carbon dioxide. Subsequent Venera probes were sent into Venus’s atmosphere, but most of them succumbed to the crushing air pressure.

The word that in paragraph 2 refers to _________.