题目

已知力F的一个分力F1跟F成30°角，大小未知，另一个分力F2的大小为F，方向未知．则F1的大小可能是（     ）       A．  F           B． F       C． F        D． F                                                                                                         —How many times have you been to China? —In fact, it is the first time I __________ here. A. was B. am C. had been　　　　　 D. have been