题目

臭氧(O3)层能吸收太阳光中大部分紫外线，从而保护地球生物，臭氧属于     A.单质         B.稀有气体      C.化合物       D.氧化物 答案：AIf only I _______ more computer skills！Then I could have gotten a job in that famous company. A. had mastered　 B. mastered C. could master　 　 D. master