单项选择题
A.a messenger
B.a stopper for nylon line
C.a U-bolt for securing a cargo whip to the winch drum
D.turns of twine around a rope end
单项选择题 A tackle by which the outer end of a boom is raised and lowered is the topping（）.
单项选择题 A stopper used in securing the ground tackle for sea that consists of a grab attached to a turnbuckle is a（）.
单项选择题 A splice that can be used in running rigging，where the line will pass through blocks，is a（）.