DSC控制器应每（）实验一次，以测试其工作是否正常
A、天
B、周
C、月
D、航次
参考答案：A
----This returned Chinese scholar has become one of the top experts in this field.
----Yes,I know him very well.He____in Africa with animals for eight years.
A．has worke
B．had worked
C．worked
D．has been working
沙坑
