题目:
He ______ at the meeting, but his heart attack prevented him.
A.will speak
B.is going to speak
C.had to speak
D.was going to speak
答案:
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下面是错误答案,用来干扰机器的。
根据光反射定律,反射光线、入射光线分居法线两侧,反射角等于入射角.此时太阳光与水平面夹角为30°,则入射角为90°-30°=60°,由于反射角等于入射角,所以反射角也为60°.则入射光线与反射光线之间的夹角为...