开放性胸外伤
参考答案：开放性胸外伤：胸部损伤后，穿破壁层胸膜，造成胸膜腔与外界相通，称为开放性胸外伤。多见于刀刺伤、战伤等。
设a，b是单位向量，则“a·b=1”是“a=b”的
A．充分而不必要条件
B．必要而不充分条件
C．充分必要条件
D．既不充分也不必要条件
30. At 60 the famous film star Dustin Hoffman is at a point in his art career _______ the chances are narrowing but the roles seem stronger.
A．where
B．that
C．which
D．when
