Kathy opened her mouth, expecting a candy and then looked puzzled she got nothing.
A. when
B. although
C. so
D. Once
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(答案→)A. when
B. although
C. so
D. Once
解析：考查连词。When 当...的时候；although 尽管；so 因此；once 曾经，一旦。
句意为kathy张开嘴巴，想要一块糖，然后当什么都没得到的时候看起来很困惑。