Acontrolled experiment tests how many variable at one time

What was the direct cause of the u.s. entry into world war ii? a. the bombing of pearl harbor b. the invasion of poland by germany c. the capture of belgium by germany d. the battle of britain

How do economists measure the success, or shortcomings, of the economy?

Ball is tossed up in the air at its peak, it stops before beginning to fall.the ball at its peak has