检验检疫机构签发的产地证书是进口国海关征收或减免关税的有效证明。（）
参考答案：对
一只船在静水中的速度为4m/s，它要以最短时间渡过一条40 m宽、水流速度为3 m/s的河．求：
(1)船过河的时间；
(2)船过河的位移大小．
_____on the grass _____singing and laughing.
A．Seated ; is some students
B．Seated ; are some students
C．Sitting ; is some students
D．Sitting ; some students are
