来自黄热病疫区的人员在入境时，必须向卫生检疫机关出示有效的黄热病预防接种证书。( )
参考答案：对
Jeff can't come to the party, because he'll help his sister with the housework. [ ]
A. must
B. have to
C. has to
D. needs to
槽液在电泳涂装过程中温度如何变化。为什么？如何解决？
