Look at the first four collisions in the video. In collision A, two people bump into each other. In collision B, two molecules (the large one is ethene, the small one is hydrogen) bump into each other. In collision C, the same two people bump into each other; and in collision D, the same two molecules bump into each other. In collisions A and B, no reaction happens. In collisions C and D, reactions do happen. Look closely at the four collisions and give two reasons why reactions happen in cases C and D but not in cases A and B.