CAREFULLY IDENTIFY THE PROBLEM NUMBER.

FOR CREDIT, INCLUDE HAND$-$CALCULATIONS OR EXPLANATION.

a$)$ vec$(a)=$hat$(e{)}_r+$rhat$(e{)}_{}+$hat$(e{)}_z$

b$)$ vec$(a)=(r-r{}^2)hat(e{)}_r+(3r)hat(e{)}_{}+$zhat$(e{)}_z$

c$)$ vec$(a)=(r-r{}^2)hat(e{)}_r+(r)hat(e{)}_{}+$zhat$(e{)}_z$

d$)$ vec$(a)=$vec$(0)$