Show that if all the S-boxes in the DES cipher are replaced by an identify function (i.e. given a six-bit input the function outputs the four least significant bits) then the resulting cipher is...

Show that if all the S-boxes in the DES cipher are replaced by an identify function (i.e.
given a six-bit input the function outputs the four least significant bits) then the resulting
cipher is easily broken. In other words, show that given a few plaintext/ciphertext pairs
an attacker can quickly recover the secret key.