Which statement about the work done by forces along a closed path is correct?
a) Both conservative and non-conservative forces do zero net work over any closed path because the starting and ending positions coincide.
b) A conservative force does zero net work, whereas a non-conservative force generally does non-zero net work over a closed path.
c) For a conservative force the work done over any closed path is the same fixed non-zero value, while for a non-conservative force it varies from path to path.
d) The net work done by any force along a closed path is directly proportional to the length of the path; therefore it cannot be zero unless the path length is zero.