生活中的下列物质，属于纯净物的是（　　）

A．食醋

B．煤

C．空气

D．纯碱

All trainees in a certain aviator training program must take both a written test and a flight test, If 70 percent of the trainees passed the written test, and 80 percent of the trainees passed the flight test, what percent of the trainees passed both tests ？（）
(1)10 percent of the trainees did not pass either test.
(2)20 percent of the trainees passed only the flight test.

A．条件(1)充分，但条件(2)不充分．

B．条件(2)充分，但条件(1)不充分．

C．条件(1)和(2)单独都不充分，但条件(1)和条件(2)联合起来充分．

D．条件(1)充分，条件(2)也充分．

E．条件(1)和条件(2)单独都不充分，条件(1)和条件(2)联合起来也不充分．