麦康凯琼脂培养基适于培养的细菌是（）
A.鼠疫耶尔森菌
B.布鲁菌
C.白喉棒状杆菌
D.肠道杆菌
E.百日咳鲍特菌
参考答案：D
治疗闭经时，应按照哪项原则（）
A.虚者补之，实者通之
B.补气养血，益经之源
C.活血逐瘀，以通为先
D.补益肝肾，以培根本
E.先攻后补，或先补后攻
A certain used-book dealer sells paperback books at 3 times dealer’s cost and hardback books at 4 times dealer’s cost. Last week the dealer sold a total of 120 books, each of which had cost the dealer$1. If the gross profit (sales revenue minus dealer’s cost) on the sale of all of these books was$300, how many of the books sold were paper backs ？（）
A．40
B．60
C．75
D．90
E．100
