What is the role of bacteria in the making of yoghurt from milk?

Question

1. to make alcohol
2. to make carbon dioxide
3. to make lactic acid
4. to make lactose coagulate

I think the answer is number 4, but i'm not so sure.

Answer 1

Bacteria eat, and the remains of lactose become lactic acid, and it is the acid which makes coagulate.

One can do add vinegar or lemon juice to make milk coagulate also.

Answer 3.

Answer 2

thanks