橙子是维生素C的良好来源。

Oranges are a good ___source of vitamin C.

专家们正试图追踪供水系统中污染的来源。

Experts are trying to track down the ___source of the contamination in the water supply.

金钱常常是年轻夫妻间紧张和争执的来源。

Money is often a ___source of tension and disagreements in young married couples.