We would like to know（ ）you can guarantee delivery within one mo

We would like to know（ ）you can guarantee delivery within one month after receipt of order.
A、that
B、when
C、if
D、which
【正确答案】：C
【题目解析】：关系代词if 和whether常在句中连接句中的宾语从句，多用于动词之后，因此本句空白处前的词是动词，所以后面的句子为宾语从句。