The pen is not as expensive as that one.

问题描述:

The pen is not as expensive as that one.
[ not as expensive as 画线.]
意思接近的.
是选 a.cheaper one
还是 b more expensive than

你a的答案有错误,应该是cheaper than
这支笔没有哪支笔贵
所以选a
not as ...as是前者不如后者的意思