Here's a problem to solve: A physicist wants to determine the speed a car must reach to jump over a ramp. The physicist conducts three trials. In trials two and three, the speed of the car is increased by 20 miles per hour. What is the physicist investigating when he changes the speed?

Among the 4 following options, which is the correct answer?

- A: the control
 
- B: the hypothesis statement
 
- C: the dependent (responding) variable
 
- D: the independent (manipulated) variable
 