For a van de graaff generator to acquire a charge on its surface what must happen?
a. positive and negative charges must be exactly equal in proportion to each other and remain so for a period of time.
b. an uncharged object must be placed nearby.
c. the metal surface must stay neutral for as long as possible.
d. positive and negative charges must be separated inside the device and one type of charge must be carried upwards to the metal sphere.