题目

Would you please speak louder, Lucy? Icatch what you said. A . can't B . oughtn't to C . needn't D . mustn't 答案：A如图所示，氢气球重8N，空气对它的浮力为20N，由于受水平风力作用，使系氢气球的绳子和地面成60°角，则绳子所受拉力为________．