Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.

Huge volcanic eruptions, called flood basalt volcanism, are believed to have caused the mass extinction at the end of the Triassic period, about 201 million years ago. But it's hard to know exactly how these eruptions destroyed ecosystems and killed off many species. Experts think the eruptions released lots of carbon dioxide, which messed up the climate. Harmful chemicals also went into the air, damaging the ozone layer, and toxic pollutants were released. One dangerous substance that came from the volcanoes is mercury, which spreads everywhere as a gas and is extremely poisonous. A recent study in Nature Communications found new evidence that global warming and widespread mercury pollution kept harming plants even after the volcanoes stopped erupting.

Scientists from the Netherlands, China, Denmark, the United Kingdom, and the Czech Republic did the study. They looked at dirt from Northern Germany using a drill-core called Schandelah-1, which covers the end of the Triassic period and the beginning of the Jurassic period. They found a lot of fern spores in the dirt, and many of them were messed up. Some had weird walls, and others showed signs of problems when they were growing, leading to tiny, strange-looking spores. Remco Bos, a PhD student at Utrecht University who led the study, was surprised by what they found. He said it's rare to see so many messed-up fern spores in dirt from a coastal area that's 201 million years old. This suggests that many ferns were struggling during that time, which is a clear sign of the mass extinction that happened at the end of the Triassic period.

( Source from Utrecht University)

The word “tricky” is closest in meaning to _______. A. straight B. simple