闭路阀是在管道工程中起()作用的阀门。
A、开启和关闭
B、调节流量
C、平衡压力
D、减小阻力
参考答案:A
—What did Mary do last Sunday?
—She _____ her homework. [ ]
A. did
B. do
C. was doing
D. have done
证券登记结算公司按照业务规则收取的各类结算资金和证券,必须存放于专门的清算交收账户,只能按业务规则用于已成交的证券交易的清算、交收,必要时强制执行。 ( )