Which year marked the introduction of the Railways in India by the British?

According to the Darwinian theory of natural selection, the rate of appearance of new forms is linked to the life cycle or the life span of an organism. Explain with the help of an example.

Phylogenetic relationship among plants $\left(P,Q,R,S\right)$ based on their shared characters is shown in the cladogram below.
Identify the plants $P,Q,R$and $S$ in the correct order.

A square loop of side $\text{'}\mathrm{a}\text{'}$ carrying a current ${\mathrm{I}}_2$ is kept at distance $\mathrm{x}$ from an infinitely long straight wire carrying a current ${\mathrm{I}}_1$ as shown in the figure. Obtain the expression for the resultant force acting on the loop.

An arrangement of three parallel straight wires placed perpendicular to plane of paper carrying same current $I$ along the same direction is shown in Figure. Magnitude of force per unit length on the middle wire $B$ is given by

A $10 \mathrm{V}$ cell of negligible internal resistance is connected in parallel across a battery of emf $200 \mathrm{V}$ and internal resistance $38 \mathrm{\Omega }$ as shown in the figure. Find the value of current in the circuit. 

Electrons are emitted by a hot filament and are accelerated by an electric field as shown in  the following figure. The two stops at the left ensure that the electron beam has a uniform cross-section.