An ammeter has 20 divisions between 0 mark and 2 A mark on its scale. The least count of this ammeter is :

A student using the same two resistors, ammeter, voltmeter and battery, sets up two circuits A and B connecting the two resistors first in series and then in parallel.

If the ammeter and voltmeter readings in the two cases be $I_1,I_2$ and $V_1,V_2$ respectively, he is likely to observe that :

Four students A, B, C and D performed the experiment to determine the focal length of a concave mirror by obtaining the image of a distant tree on a screen. They measured the distances between the screen and the mirror as shown in the diagrams given below :

The correct value for focal length will be obtained by the student :