His younger sister may already be in high school now, in case this picture book is too ch

网友回答

A. that
(答案→)B. which
C. whose
D. what
解析：考察定语从句。本题的定语从句用which case一起来指代前面一句话的情景，句意：他的妹妹已经上高中了，在这种情况下，这些图片书对她来说就太幼稚了。That不能放在介词的后面；whose在句中作定语。What不能引导定语从句。故B正确。