OG 语文分册 CR67

xingym

A complelling optical illusion called the illustion of velocity and size makes objects appear to be moving more slowly the larger the objects are.  Therefore, a motoriset's estimate of the time available for crossing a highway with a small car approaching is bound to be lower than it would be with a larger truck approaching.

The conclusion above would be more properly drawn of it were made clear that the

A) truck's speed is assumed to be lower than the car's
b) truck's speed is assumed to be same as the car's
c) trck's speed is assumed to be higher than te car's
d) no tie choice
e) another no tie choice

答案是B，
看题干应该是问assumption, 应该是找一个必要条件， B是必要条件吗？ 取非之后好像不能推论出论据为错啊？