which filesystem[s] can safely be mounted readonly?

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

A.
/var, /usr

B.
/var

C.
/usr, /, /tmp

D.
/usr

E.
/tmp

Explanation:
/tmp and /var both need to be written to. /usr is the most static of the directories (excluding
/usr/local)