You have two boxes of colored pens. the first box contains a red pen, a blue pen, and a green pen. the second box contains a yellow pen, a red pen, and a black pen. what is the set that represents all the pens?
a.{red pen, blue pen, green pen}
b.{red pen}
c.{red pen, red pen, green pen, yellow pen, black pen}
d.{red pen, blue pen, green pen, yellow pen, black pen}