While these pointers to answer causal questions are helpful tips, one must remember that the key role of a strengthener is that it makes the conclusion more likely. This is because, unless you understand this, you will not be able to apply the rules well.

This argument: This argument concludes that smoking contributes to depression in teenagers. The conclusion is based on the data from the study that states that people who smoke are 4 times as likely to be depressed.

When you see a question like this, you should ask yourself.. is it likely that people who take up smoking are depressed in the first place. If this is so, then it is depression that causes people to take up smoking not the other way round. Therefore, a strengthener would be something that discounts the possibility described above. Choice A does exactly that.

Think of a similar passage..

Students who take remedial classes are twice as likely to fail than people who dont. Hence, remedial classes cause students to fail.

In the argument above, one would ask.. Are students who join remedial classes more likely to fail in the first place.