In order for a magnetic force to exist between a source charge and a test charge __________ . a. both the source charge and the test charge must be moving. b. the source charge must be moving, but the test charge must be stationary. c. the source charge must be stationary, but the test charge must be moving. d. both the source charge and the test charge must be stationary. e. the source charge and the test charge must both be uniform currents.
