1. 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, red pen, and a black pen. what s 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}