Tom was walking along the riverside when he heard someone calling him from behind. He turn

问题补充：

Tom was walking along the riverside when he heard someone calling him from behind. He turned around found nothing.

A. but

B. and

C. so

D. or

答案：

(答案→)A. but

B. and

C. so

D. or

解析：句意：Tom 正沿着河边散步这时他听到有人从后面喊他。他转过身来，但是什么也没发现。根据句意可知，此处应表示转折的关系。and和，表示承接；so 因此，所以，表示因果；or 或者，表示选择。故选A。