Chemists and physicists often have to manipulate and control a variable that they want to study. Scientists who need to control a variable must perform which type of scientific investigation? A. experimental
C. comparative
B. free exploration
D. observational

Sandy is building a small toy car. he wants to use a balloon to power the toy car. he fills a balloon with air and then attaches a straw to the balloon. he tapes the balloon-straw combination to the car and then releases the end of the balloon. the toy moves forward as the air from the balloon comes out the back of the straw. what can sandy do to make the toy car move faster? a) use less air in the balloon. b) blow up the balloon more. c) use a longer straw. d) use larger tires.