2H、13C、15N、18O、34S等常用作环境分析指示物，下列对它们的说法正确的是
A．34S原子核内的中子数为16 B．13C和15N原子核内的质子数相差2
C．16O与18O互称同位素 D．2H的核外电子数比1H多一个
She brought with her three friends, none of ______ I had ever met before. 
| A．them | B．who | C．whom | D．these |
I tried phoning her office, but I couldn’t _______. 
| A．get along | B．get on | C．get to | D．get through |
His sister left home in 1998, and _______since. 
| A．had not been heard of | B．has not been heard of |
C．had not heard of | D．has not heard of |
What do you mean, there are only ten tickets? There _____ be twelve
| A．should | B．would |
C．will | D．shall |
Could I speak to ______ is in charge of International Sales please?
| A．who | B．what |
| C．whoever | D．whatever |