单项选择题
A fire starts when the following （） factors come together.
A.combustible material B.ignition source C.oxygen D.A and B and C
单项选择题 （） can be operated independently or locked together and operated as a twin-job cranes of double capacity.
单项选择题 Some cranes are mounted in pairs on a common platform which can be rotated through（）.
单项选择题 Deck cranes require（） time to prepare for working cargo than derricks.
