Four 1000Ω resistors are connected as shown in the following circuit. Two of the resistors are changed to 900Ω. Changing which pair of resistors would make a current flow in the ammeter?

Answer:

4) C

No calculations needed for this question. 

Current follows the path with least resistance. 

A – Wrong
If resistance of p and q becomes lower, then the current will only flow through the top branch. It will not go down to the ammeter from the top branch.

B – Wrong
If resistance of p and q becomes lower, then the current will split equally between top and bottom branch. It will not flow through the ammeter.

C – Correct
If resistance of r and q becomes lower, then the current will flow through the top branch first. It will then go down to the ammeter from the top branch in order to reach r.

D – Wrong
If resistance of r and s becomes lower, then the current will only flow through the bottom branch. It will not go up to the ammeter from the bottom branch.