如图，长为L的细绳拴有一个质量为m的小球，当小球绕悬挂点O摆动经过最低点时，细绳受到的拉力为3mg。若在小球经过最低点时用光滑细杆挡在绳的中点，这时细绳受到的拉力的大小为：（ ）
What _____ it is to go horse riding __________ weather!
________ great scientist Qian Xuesen is!
（）I think _______ necessary for you _______ breakfast.
The workers were made _____ the whole day.
He often plans_________ a very relaxing vacation. A. having B. have C. to have
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