$\alpha$ -Keratin Found in Nails Has More Strength Than $\alpha$ -Keratin Found in Hair Due to the Presence of More

$\alpha$ -Keratin found in nails has more strength than $\alpha$ -keratin found in hair due to the presence of more ___________.

A) hydrophobic interactions
B) salt bridges
C) ( $\alpha$ -helix)
D) disulfide bridges
E) pleated sheets