根据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凭证。( )
参考答案：对
—Xiao Wang, will it take to fly to Guangzhou?
—Sorry,l don't know. [ ]
A. how far
B. how soon
C. how many
D. how long
简述应激的定义。
