Which of the following statements are CORRECT about the function of fetal bovine serum in animal cell culture?
- lt stimulates cell growth
- It enhances cell attachment
- lt provides hormones and minerals
- It maintains $pH$ at $7.4$
- $P$ and $Q$ only
- $P$ and $S$ only
- $P, Q$ and $R$ only
- $P, Q, R$ and $S$