问题
选择题
We had great fun _____ in the park. [ ]
A. play
B. plays
C. playing
D. Played
答案
答案：C
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We had great fun _____ in the park. [ ]
A. play
B. plays
C. playing
D. Played
答案：C
的透明长方体放在空气中，矩形ABCD是它的一个截面，一单色细光束入射到P点，入射角为θ，AP=
AD，AD=d，求：