On a system with separate partitions for /, /usr, /var, /tmp, which filesystem[s] can safely be mounted read-only?

On a system with separate partitions for /, /usr, /var, /tmp, which filesystem[s] can safely be mounted read-only?

A.
/var, /usr

B.
/var

C.
/usr, /, /tmp

D.
/usr

E.
/tmp