7. A rope of mass M and length L hangs vertically.What is the time T needed for a pulse to travel from the bottom end to the support? (A) (B) (C) (D) 2 (Lig)； (E) None.

4. If the energy needed to send an object (initially at rest on the earth's surface) to a maximum height H is a half of that for sending it into an orbit at the same height, what would be the value of H in terms of RE (the radius of the earth)? Ignore the earth's rotation. (A) 1/4； (B) 1/3； (C) 1/2； (D) 2/3； (E) None.