In an experiment Investigating how far model airplanes with different shapes can travel, which of the following are variables that need to be
controlled?
a. type of wood used; mass of the planes, glue used: air currents and
breezes
b. whether the shapes look like real airplanes: how old the models are
c. what time the test starts: the time it takes for each test
d. whether the models land smoothly or become damaged during the
test