Which one of the following has the longest covalent bond distance?

Which one of the following molecular hydrides acts as a Lewis acid?

Which of the following is the most effective in causing the coagulation of a ferric hydroxide sol?

If ${x^y} = {y^x},$ then ${(x/y)^{(x/y)}} = {x^{(x/y) - k}},$ where $k = $

The radius $R$ of the soap bubble is doubled under isothermal conditions. If $T$ is the surface tension of the soap bubble,the work done in doing so is given by: (in $\pi R^{2} T$)

Two thick wires and two thin wires,all of the same material and same length,form a square in three different ways $P$,$Q$,and $R$ as shown in the figure. With the current connection shown,the magnetic field at the centre of the square is zero in which cases?