Interlocks are used to ensure that operations in a chemical plant are performed in the proper sequence. Interlocks can be mechanical or electronic. In many cases. they can be as simple as a lock and key. Specify the simplest mechanical interlock capable of achieving the following functions:

a. A valve cannot be closed until a furnace is shut down.

b. Two valves cannot both be closed at the same time.

c. A valve must be closed before a pump is started.

d. The feed to a reactor cannot be started until the reactor vessel stirring motor is activated.