单项选择题
Chain slings are suitable for heavy slender articles，such as（）.
A.timber or steel rails B.locomotives or boilers C.railway passenger coaches D.motor vehicles
单项选择题 Chafing gear（）.
单项选择题 Before welding in a tank that has carried petroleum products，the tank must be certified by （）.
单项选择题 Before welding can be done in a tank that has carried petroleum products，a certificate must be obtained from（）.
