何时制定扭转试验无效？
参考答案：
若试样在离夹头2a范围内断裂，且未达到规定的扭转次数则试验无效。
Don’tletthesadnessofyourpastandthefearofyourfuture ___ thehappinessofyour present.
A．to ruin
B．ruin
C．ruining
D．ruined
