What is the BEST conductor of electricity?（）
A. Carbon dioxide
B. Distilled water
C. Fresh water
D. Salt water
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发展中国家由于技术水平低下，与发达国家的贸易竞争总处于劣势。（）
A. 正确
B. 错误
The exhaust ports of a diesel engine using the cross-flow scavenging method are opened and closed by the （）
A. reciprocating motion of exhaust valves
B. rotary motion of the camshaft
C. reciprocating motion of the piston
D. developed differential
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