下列晶体中由原子直接构成的单质有[ ]
A．白磷
B．氦
C．金刚石
D．金属镁
BC
—How was Robert's cooking?
—Oh, pretty good. I was quite _____.
A．admired
B．interested
C．impressed
D．inspired
试述马尔萨斯的经济危机理论并作评价。
