Two identical pulses, one inverted, travel in opposite directions along a stretched string, and interfere destructively when passing. It takes 8.3 seconds for the two pulses to completely pass by each other. Which of the following is true during the passage?

A. There is an instant at which the string is completely straight.
B. When the two pulses interfere, the energy of the pulses is momentarily zero.
C. There is a point on the string that does not move up or down.
D. There are several points on the string that do not move up or down.
E. A and C are both true.
F. B and D are both true.