Electric cars are becoming more popular. They cause less polluti

Electric cars are becoming more popular. They cause less pollution and save more energy, (compound sentence)
【正确答案】：Electric cars are becoming more popular,for they cause less pollution and save more energy.
【题目解析】：compound sentence的意思是并列句，for表示“因为”、“由于”， 连接并列句时，只表示一种推测或解释，后面的分句通常表示间接的推测原因，前句表示结果，后句表示原因。例如： The electric current must have been turned off, for the light went out. 想必是停电了，因为灯灭了。这里我们用for来连接两句并列句，表示电动汽车越来越流行，因为电动汽车污染少且节能。