—I________as a cashier for 10 straight years, but I never had a sense of achievement.

—Is that why you set up your own business?

A．worked

B．have been working

C．would work

D．have worked

污水中的氮元素主要以（）形式存在

A.对有机氮和氨氮

B.有机氮和凯式氮

C.有机氮和无机氮

D.凯式氮和无机氮