气门弹簧弹力变弱时，则气门容易跳动。
参考答案：对
销售折让和销售退回均会影响企业的营业成本。（）
--- Daddy, I don’t want to go to Jimmy’s birthday.
--- You had better go. ____ you make a promise, you have to keep it.
A．Once
B．Even if
C．Unless
D．In case
