制特种玻璃时原料中添加了硼砂(Na2B4O7·10H2O)，其中硼的化合价为 [ ]
A．+1
B．+2
C．+3
D．+4
答案：C
— How many students are there in the room?
— Twenty _____, I'm not sure. [ ]
A. or so
B. or rather
C. about
D. more than
均相恒沸物在低压下其活度系数之比Y1/Y2应等于（）与（）之比。
