b.

A crocodile's head is longer than An alligator's head

c.

Crocodiles are more aggressive than alligators

d.

An alligator's skin is darker than A crocodile's skin

e.

Crocodiles are more common  than alligators.

f.

An alligator's mouth is wider than A crocodile's mouth

$\text{b. A crocodile's head is longer than an alligator's head}$

$\text{c. Crocodiles are more aggresive than alligators}$

$\text{d. A alligator's skin is darker than a crocodile's skin}$

$\text{e. Crocodiles is more common than alligators}$

$\text{f. A alligator's mouth is wider than a crocodile's mouth}$