Commercial use of microorganism - class-X

Study the following statements regarding lactic acid bacteria (LAB) which are used to convert milk into curd.
(i) They produce adds that coagulate and partially digest the milk proteins.
(ii) A small amount of curd added to the fresh milk as an inoculum contains millions of LAB, which at suitable temperature, multiply and convert milk into curd.
(iii) Conversion of milk into curd improves its nutritional quality by increasing vitamin $B _{12}$.
(iv) LAB may result in acidity in the stomach of human beings.
Which of the given statements are correct?