看了曹老师的解答，我作一补充：

Not only is smoking bad for your health,it's the source of social  problems in developing countries because, since profit from tobacco is very high, tobacco has taken up the best food-growing areas, leading to food shortages.

【翻译】吸烟不仅有害健康，也是发展中国家社会问题的根源，因为烟草的利润很高，烟草占据了最好的粮食种植区，导致粮食短缺。

【简析】这是一个复杂的主从复合句，在because从句中又包含了一个“主从复合句”： since profit from tobacco is very high, tobacco has taken up the best food-growing areas, leading to food shortages. 其中，现在分词leading to food shortages 作结果状语。

1. since 是后面主句的原因，实际是提供了一种理由（烟草的利润很高），但远没有because强烈。

2. 两个从属连词碰到一起时，之间经常加逗号，以区别主次关系和所属关系。又如：

I didn't give my name because if I didI thought you might not have come.

It's always good to try new things because, when you are trying new things you are growing.

为了理解方便，我们把becasue 句中的那个从句放到句末：

I didn't give my name because I thought you might not have come if I did.

It's always good to try new things because you are growing when you are trying new things.