What is conductivity? Give its SI unit

What is resistivity?Give its unit.

What is pigeon's milk? Give its function.

Define potential gradient. Give its SI unit?

The decay rate of a radioactives ample is called its activity. What is the SI unit of activity?

What is magnesia cement? Give 'its composition?

What is a zener diode? Give its symbol.

One can determine the direction of electric field around a stationary charge with the help of electric field lines. Br What do you understand by the term .electric flux.? Give its Sl unit.