C, none of the other answers are true or make sense.
Answer:
The people have invaded Christian lands, and have depopulated themExplanation:
^^^^
I cannot do this entire thing for you, but I can help.
A cause is the thing that forces something else to happen. So, that could be anything. The effect is what comes after the event.
For example, the driver was texting while driving. That is the cause. He hit a girl. That is the event. He girl was injured. That is the effect.
Hope that helps!