阅读下列短文，从每题所给的四个选项([A]、[B]、[C]和[D])中选出最佳选项。A To make the play dough recipe you will mix all ingredients in a saucepan. Cook over medium heat, stirring until stiff. Allow the mixture to cool, then knead the dough by rolling it hard and squeezing it in your bands. The play dough must be stored in a zip lock bag until you use it or it will dry out and become hard. To make the ornaments you will need Christmas cookie cutters in all different shapes. Roll out the play dough and cut the dough with the cookie cutters. Place a hole at the top of the shape for a hanger to go through. You can use pretty ribbon, yarn, or siring to pull through the hole for the hanger. These are nice gifts to give to your teachers, parents, and friends at Christmas time to bang on their Christmas trees and they are so special because they were made by you! Why must you use a bag to keep the dough in

A. So it will not dry out.
B. To keep it dry.
C. To keep it pretty.
D. To use ribbon.