单选题 It is absolutely necessary for us to have sufficient oxygen if we ______ to go on living.
A. are B. were C. had D. having

【正确答案】 A
【答案解析】[解析] if we are to go on living=if we want to go on living“如果我们想要继续生存的话”。本题不属于纯假设的虚拟条件句。