69. The human growth hormone, made by the pituitary gland, is secreted during sleep in higher concentrations than when awake.
(A) during sleep in higher concentrations than when awake
(B) when sleeping in higher concentrations than waking hours
(C) in higher concentrations during sleeping than waking
(D) in higher concentrations during sleep than during waking hours（D）
(E) in higher concentrations when asleep than when awake
答案为D， 可是我觉得E， when asleep than when awake也挺对称的呀。是不是asleep 和awake只能做表语be asleep和be awake 而不能用在when后面呀？请大牛指教。
when asleep than when awake
是个省略主语和be动词的省略结构。asleep和awake的逻辑主语即为句子主语，也就是when the human growth hormone is asleep，human growth hormone又不能sleep，逻辑上是不对的
谢谢ranning, 解释非常到位！
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