An astronaut Alice in a spaceship leaves earth at a speed of 3c/5. When a clock on the spaceship says 1 year has elapsed, Alice sends a light signal back to earth. (a) According to Earth's reference frame, when was the signal sent? (b) According to Earth's reference frame, how long after the spaceship left did the signal arrive on earth? (c) An observer Bob on earth will reply by sending a light signal back to the spaceship when he receives Alice's signal. According to Alice, how much time after sending her signal will elapse before she receives the reply from Bob?