对于氟利昂制冷系统，当( )时，采用双级压缩。
A．PK/PO≥10 B．PK/PO＜10 C．PK/PO≥7 D．PK/PO＜7
参考答案：A
He doesn’t know _________ English because he has studied it for only ________
weeks
A．much; a few
B．little; few
C．few; a little
D．a few; a bit
20世纪90年代以来，国际资本流动的主要特点有哪些？
