In trying to solve the pendulum problem, formal operational adolescents usually ________. Question 7 options: a) test changes in string length and changes in weight together before testing them in isolation b) focus on only the weight of the object on the pendulum, and not the length of the string c) identify relevant variables and test each one separately and, if necessary, also in combination d) are able to determine the effect of changing the force of the initial push, but not other changes