He performed a streak test, in which a piece of the mineral is rubbed across a piece of unglazed porcelain in order to determine the color of the mineral in powdered form.
Answer:

Explanation:
In series combination, the equivalent resistance is given by :

Let the identical resistors be R. We have, 
So,

So, the resistance of each resistor is
.
B. Because they are used to see through objects