(b) List all of the permutations of (a, b, c, d) when the elements are taken two at a time. (Select all that apply.)
{d, a}
☐ {a, e)
□ {a, c)
{b, d)
{c, a}
{b, a}
(b, c)
□ {d, c)
{a, a}
{a, b)
□ (c, d)
{d, d)
{d, b}
□ {c, b)
{b, b)
(d, e)
{a, d)