What were the main reasons of the fall of Soviet Union?

I'm in 9th grade and I find history quite interesting. I've study the events before the fall of USSR and it's hard to pinpoint what caused the fall of USSR. Some of the events I've covered are:<br><br>Soviet-Afghan war (I've read that the war was quite costly)<br><br>Decline of oil prices (since the USSR was a net exporter of oil)<br><br>Economic problems of USSR (I've read that USSR was very costly and they had to import wheat)<br><br>Death of three premiers (I'm not sure if this had any effect on the Soviet power)<br><br>Soviet-Afghan war and rise of religion in the Central Asian republics<br><br>Rise of the colour revolutions and decline of communism in Eastern Europe<br><br>So I just wanna know what caused the fall of USSR.