下列有关脂肪栓塞的机制，正确的是()
A．药物学说
B．机械或血管外源学说
C．化学或血管内源学说
D．梗阻学说
E．开放学说
参考答案：B, C
— Shall we go skateboarding after school?
— Sorry, my parents said that I _____ be at home early. [ ]
A. need
B. must
C. may
拍摄大件商品的操作流程是？
