如图所示表示一交变电流随时间变化的图象，此交变电流的有效值为
A．3A
B．A
C．A
D．A
答案：A
设集合A={1，2，3}，B={2，4，6}，则A∩B=______．
Which of the following statements regarding diminishing marginal returns is most accurate（）
A. As the quantity produced rises, costs begin to rise at a decreasing rate.
B. As the quantity produced falls, costs begin to rise at a decreasing rate.
C. As the quantity produced rises, costs begin to rise at an increasing rate.
A 
A 
A
A A A