Answer the following questions related to the concept of escape velocity.
a. (i) Define the term escape velocity of a body from the surface of a planet.
b. (ii) The gravitational potential energy U of a body at a distance r from the centre of Earth is taken to be negative. State the reason for assigning this negative sign.
c. (iii) Explain why the total mechanical energy of a body is chosen to be zero when it is at an infinite distance from the attracting planet.