湿热带型电机可装有防潮加热器。（）
参考答案：对
乳稠计的读数为20时，相当于（）
A、相对密度为20
B、相对密度为20%
C、相对密度为1.020
D、相对密度为0.20
Space exploration began in the 1960s, and since then about five hundred people _______ into space. [ ]
A. are traveling
B. traveled
C. had traveled
D. have traveled
