脊髓压迫症的最常见病因为（）

A.脊椎疾病

B.脊髓髓内肿瘤

C.椎管内脊髓外肿瘤

D.急性脊髓炎

E.脊髓空洞症

When launching a lifeboat，the tricing pennants should be released（）.

A.before the boat is lowered from the stowed position

B.as the boat-fall blocks break clear of the davit head

C.before the boat is lowered from the embarkation level

D.after the boat is released into the water