单项选择题

分子取向作用会使塑料力学性能各向异性，下面结论哪个正确？
A、顺着分子定向的方向上的机械强度大于与其垂直方向上的，伸长率则相反
B、顺着分子定向的方向上的机械强度小于与其垂直方向上的，伸长率相等
C、顺着分子定向的方向上的机械强度和伸长率总是大于与其垂直方向上的
D、顺着分子定向的方向上的机械强度与其垂直方向上几乎相等，伸长率不同

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