儿童最容易受外伤的牙齿是（）
A.乳切牙
B.乳尖牙
C.恒切牙
D.乳尖牙和恒尖牙
E.乳切牙和恒切牙
参考答案：E
— I have got a headache.
— No wonder. You ______ in front of that computer too long. [ ]
A. work
B. are working
C. have been working
D. worked
塑料三维土工网的代号（）。
A.NSS
B.NSP
C.NJS
D.NJP
