Find the wrong statement from the following about the equation of stationary wave given by $Y=0.04 \cos (\pi x) \sin (50 \pi t) \mathrm{m}$ where $t$ is in second. Then for the stationary wave.

A convex lens of focal length ' $f$ ' is placed in contact with a concave lens of the same focal length. The equivalent focal length of the combination is

Two light balls are suspended as shown in figure. When a stream of air passes through the space between them, the distance between the balls will

The range of an ammeter of resistance ' $G$ can be increased from ' $I$ ' to ' $nI$ ' by connecting