To add BigInteger b$1$ to b$2,$ you write $\_\_\_\_\_\_\_\_\_.$

A$.$

b$2=$ b$2.$add$($b$1)$;

B$.$

b$2=$ b$1+$ b$2$;

C$.$

b$2.$add$($b$1)$;

D$.$

b$2=$ add$($b$1,$b$2)$;