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

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A. that
(答案→)B. which
C. whose
D. what
解析：B　考查定语从句。句意：他的妹妹现在可能已经上高中了，那样的话这本图画书对她来说太幼稚了。在定语从句中，有时为了表达的意思更清楚，可用“which＋名词”引导定语从句，本句中为更清楚地表达“在他妹妹已经上高中的那种情况下”，故用in which case。