Read the following passage and mark the letter A, B, C, or D to indicate the correct answer that best fits each of the numbered blanks.

A tornado does not necessarily need to be visible; however, the extremely low pressure caused by the high wind speeds and rapid rotation usually causes water vapor in the (21) _____________ to condense into a visible condensation funnel.

Tornadoes can happen in nearly any part of the world. In the United States, a tornado has been (22) _____________ in all states. The middle part of the United States is nicknamed "Tornado Alley" for the large number of tornadoes created there.

A tornado is a tube of (23) _____________ spinning air that touches the ground. Wind inside the tornado spins fast, but the actual "circle" of wind around them is massive. This makes tornadoes very (24) _____________.

Tornadoes mostly happen during strong thunderstorms. They cause a lot of damage to anything in the path. Tornadoes (25) _____________ ranked on the Fujita scale, from F0 to F5. The F0 has the lowest wind speed, and the F5 has the highest one.

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Hoi An ______________ an important trading centre in Southeast Asia. A. used to be B. used to being C. was used to be D. use to be