An electric motor must have which of the following in order to work?
a. galvanometer
b. power source
c. secondary coil
d. primary coil

B. Power Source

Explanation:

Electric motor is a device which convert the electrical energy into mechanical energy.

In this device we have a coil of wire with N turns. When current flows through the coil then it will become a coil with finite magnetic moment.

It is given as

now this coil will experience the torque on the coil which is given by formula

due to this torque the coil start rotating so here main working of motor is to convert electrical energy into mechanical energy

So we provide the Power source to the coil from which it will get the electric current and then convert it into mechanical energy

so correct answer will be

B. Power Source

answer: the metamorphic rocks are formed when they are exposed to high pressure and high temperatures. this happens when one of the earth's plates comes in contact with the other plate by traveling in opposite directions, straight into each other or slipping under one another. the friction that is caused by this activity results in   melting of rocks and when it cools they are known as metamorphic rocks. this process needs a controlled amount of pressure and temperature otherwise it may turn into magma.