Great Salt Pond on Block Island is（）.

A.entered through a dredged cut

B.not accessible in easterly gales

C.available for vessels up to a maximum draft of 8 feet (2.4m)

D.not affected by the tide

中滑板丝杠与（）部分由前螺母、螺钉、中滑板、后螺母、（）和楔块组成。

A.圆锥销

B.丝杠

C.圆柱销

D.光杠