Some 25,000 people were reported ______ in the ever recorded worst

earthquake in Iran which occurred late this month.

A．were killed

B．to be killed

C．killing

D．to have been killed

市售化肥硝酸铵的化学式为（NH₄NO₃）．试求：

（1）硝酸铵的相对分子质量为______；（2）硝酸铵中N元素的质量分数______．