紫外线对皮肤的影响包括（）。
A.免疫增强
B.光老化
C.诱导肿瘤
D.光敏性皮肤病
参考答案：B, C, D
—Tony, come down and help me with the heavy box!
—Come on! I _____ all day long. Get Peter to do that! [ ]
A. work
B. had been working
C. had worked
D. have been working
虚实错杂
