请教prep－129

waterer

129. 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

请教选B错在哪里？