在经济学巾，把能满足人们对某种需要的效用的价值称为( )。A．使用价值 B．交换价

设等差数列{a_n}的前n项和为S_n，S_m－1＝－2，S_m＝0，S_m＋1＝3，则m＝ (    )

A．3

B．4

C．5

D．6

An investor entered into a margin purchase transaction. He purchased on margin 100 shares of stock priced at $ 60 per share. The investor borrowed the maximum amount allowed by the initial margin requirement of 50 percent, and the maintenance margin is 30 percent. Under the requirement of margin purchase, the lowest price per share at which the stock could sell before the investor would receive a margin call is closest to:()

A. $30.

B. $40.

C. $43.

过度捕捞导致生态系统的结构、功能退化。

原发性腹膜炎指（）

A.腹内脏器炎症所致的腹膜炎

B.由腹腔内脏器创伤引起的炎症

C.腹腔内夫明显的原发病灶的腹膜炎

D.腹腔手术污染引起的腹膜炎

E.腹腔内出血所致的腹膜炎

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