Researchers wanted to determine if pesticide Z was effective at killing grasshoppers that typically invade cornfields. Two ten-acre cornfields located 1 mile apart were chosen. The cornfield to the west was sprayed only with water. The cornfield on the east received the same amount of water with a 10% solution of pesticide Z. Samples of the numbers of grasshoppers in the field were made every week for 10 weeks. They found that the number of grasshoppers doubled after ten weeks in the west field treated only with water. They also found that the number of grasshoppers declined by 50% after 10 weeks in the East field treated with water and pesticide Z. Which field was the control group?

Control group is also known as comparison group. According to the definition, in an experiment a control group is that type of group which is used as a way to make it sure that the experiment is actually working. During the experiment two groups are formed, one out of these is called experimental group and the other is called the control group. The experimental group receives the treatment that is being tested in the experiment while control group receives only a standard treatment or no treatment.

In the question the west part of the cornfield was sprayed only with water which means this group got only the standard treatment to this is control group. The east part was sprayed with pesticide mixed with water so this is the experimental group.