[求助] 语法题目一道

Some patients who do not respond to therapies for depression may simply have received inadequate treatment, having, for example, been prescribed a drug at a dosage too low to be effective or having been taken off a drug too soon.

A.     having, for example, been prescribed a drug at a dosage too low to be effective or having been

B.     having, for example, a drug prescription that was ineffective because the dosage was too low, or being

C.     as, for example, having too low of a dosage of a prescribed drug for it to be effective, or being

D.     when they have, for example, been prescribed too low a drug dosage to be effective, or were

E.      for example, when they have a drug prescription with a dosage too low to be effective, or been

答案：A。

记得XDF白大侠说过，having been done 这种分词结构是错的，要改成从句的。这里确实答案。不解？？

请指教！！谢谢！！