————the drill is,it is very useful in improving your oral English.

C. 考察as引导的让步状语从句，全句意思是：尽管你觉得练习很简单，但它确实能提高你的口语。

as引导让步状从时，意为“即使，即便”；这时as不能置于句首，要放在所强调部分的后面，其结构为：被强调成分+as+其他正常语序（主语+谓语）
这里的被强调成分就是simple(简单)，语序为：simple+as+you+consider

A项可以这么改：simple （as）you consider...
B项可以这么改：Although(Though) you consider the drill is simple...
D项可以把as和simple颠倒位置

让步状语从句中的倒装
形容词；副词；名词（不加冠词）；分词；动词 + as/though + 主 + 谓
Hard as/though he studies, he cannot pass the exam.
Child as/though he is, he knows a lot.
Absorbed as/though he was in the book, he did not
Try as/though he would, he might fail again.

as 引导状语从句时，若将其后的adj. adv或n.提前时，需要倒装。前两者直接放置句首，但名词前无冠词。如：Child as he was, he was brave.

倒装吧