单选题 If you do something on ______, you do it because you suddenly want to, although you haven"t planned to.
【正确答案】 A
【答案解析】[解析] 名词词义辨析。根据句子结构，空格应填入与suddenly，haven"t planned to意思相近的名词。选项中impulse(冲动)符合句意。故答案为A。