[误] My cousin Mary often feels lonely; I supose it\'s because she is a single child.

[正] My cousin Mary often feels lonely; I suppose it\'s because she is an only child.

注：only child 是一个固定搭配，而且 only 强调“唯一，只有一个”，而 single 常用于指“单身的”。例如：Mary is single(玛丽还没有成家)。