（）is the main center-line structural member，running fore and aft along the bottom of a ship，sometimes referred to as the backbone.

A band or collar on the top end of a boom to which the topping lift，midships guy，and outboard guys are secured，is called the（）.

A.collar band
B.guy band
C.pad eye collar
D.spider band

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A continuous watertight bulkhead is normally also a(n)（）.

A.structural bulkhead
B.exterior bulkhead
C.centerline bulkhead
D.joiner bulkhead

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