Two groups wanted to determine which size of parachute, when released, fell to the ground the slowest. each group's design and results are shown in the table below. each group designed their parachutes as follows: design and results by group group 1 6 20 cm pieces of string 1 50 g package 24 cm round plastic parachute results 6 seconds group 2 6 20 cm pieces of string 1 50 g package 12 cm round plastic parachute results 3 seconds which of the following explains why this exercise represents a carefully designed experiment? a. they used the same mass of package in all the trials. b. they altered only one variable in their design and testing. c. they used the same materials when making each parachute. d. they confirmed that the smaller the parachute, the faster it falls.