In which case is there an induced electromotive force?
a. a permanent bar magnet is held inside a coil of wire
b. field line's from earths magnetic field cross the area of a coil of wire
c. an electromagnet operating on direct current is held a second coil of wire
d. an electromagnet operating on alternating current is held inside a second coil of wire

When can a theory be modified if a new type of technology allows for new observations that raise new questions? a-immediately, while the questions about the theory are being asked b-after new hypotheses related to the theory are tested in experiments c-only after the new observations disprove all parts of the theory d-while scientists begin to think about how the theory could improve