Which set is closed under the operation multiplication?

a.
{1, 2, 3}

b.
{0, 1}

c.
{0, 1, 2}

d.
{1, 2}

I have another set of choices here:

A.{0.5, 1, 2} 

B.{1, 2, 4} 

C.{1, 2, 3, 4, 5} 

D.{0} 

If you multiply two numbers from a closed set the answer is in the set. 
So A is not closed because 2 x 2 = 4 which isn't in the set. Only 0.5, 1, and 2 are in set A. 

Try multiplying the largest numbers in sets B, C, and D by themselves. You will soon discover the closed set.