JWK型经济型数控机床系统电源切断后，必须等待30s以上方可再次接通电源。允许连续开、关电源。
参考答案：错
The Russian rocket designer has died. For many years, he ______ as an important assistant to the father of Soviet space program.
A．serves
B．was serving
C．served
D．has served
阿米巴病的主要传染源（）
A.症状明显的患者
B.受感染的动物
C.恢复期患者
D.无症状包囊携带者
E.慢性患者
