曝气池内混合液的污泥浓度(MLSS)是活性污泥处理系统重要的设计与运行参数，确定这一参数时，应考虑的因素有( )。

A．供氧的经济和可能性

B．活性污泥的凝聚沉淀性能

C．沉淀池与回流设备的造价

D．供氧设备的型号

Selection to participate in a top executive-education program is an important rung on the ladder to top corporate jobs. U. S. corporations (1) billions of dollars in this form of management development -- and use it to (2) and train fast-track managers. Yet one (3) of executive education found that less than 5% of the managers (4) to these high-profile programs are women -- and minorities are terribly (5) as well.

The numbers are (6) . In regular business (7) usually paid for by the participant, not an employer -- there are plenty of women and minorities. Women, for example, (8) for about 30% of MBA candidates. Yet in the (9) programs paid for by corporations that round out a manager’s credentials at a (10) career point, usually at age 40 or 45, companies are making only a (11) investment in developing female and minority executives. A case (12) point: Only about 30% of the 180 executives in Stanford’s recent (13) management program were women.

Most companies say these days they are (14) hiring and promoting women and minorities-- and there are some (15) trends in overall employment and pay levels so why are companies (16) the ball when it (17) executive education The schools (18) that they are neither the cause of nor the cure for the problem. A Harvard Business School dean figures that companies are (19) of sending their female executives (20) they don’t want to lose them to competitors.

20（）

A.that

B.while

C.because

D.But