Random question?

What causes a person to have blue eyes?

Random question?

Henry and Emily are building a tree house. Which tool should be used to measure the height of the door?

Random question?

Researchers are developing new types of crop plants with desirable characteristics such as disease resistance and drought tolerance. Crossing plants so that the next generation will express desirable traits is best described as

Random question?
Which of these groups contains single-celled organisms?