Which of the following answer is correct. Select one answer that is most appropriate. Partial credits may be given.

a$.$

DES is vulnerable to brute$-$force attacks.

b$.$

Multiple DES, in particular, $3$DES$/3,$ effectively extends the key length to $168$ bits, and so far resists brute$-$force attacks.

c$.$

$2$DES$/2$ is vulnerable to meet$-$in$-$the$-$middle attack, which in this case is particular kind of brute$-$force attack.

d$.$

The meet$-$in$-$the$-$middle attack on $2$DES$/2$ is hard to extend to $3$DES$/2.$

e$.$

All of the above.

f$.$

$3$DES$/3$ is not secure at all because it is equivalent to a single DES with a different key, which is then vulnerable to brute$-$force attacks.

g$.$

$3$DES$/3$ is vulnerable to meet$-$in$-$the$-$middle attacks.

h$.$

None of the above