Imagine how excited we were       that our daughter had been employed by the well-known company.

A．learning

B．to learn

C．having learned

D．to be learning

流行病学分析中的混杂因素不具备下列哪项条件？（）

A、与疾病有关联

B、与暴露因素有关联

C、在分析的各组中分布不均匀

D、是病因链上的中间环节