Which of the following has the longest $C-O$ bond length? (Free $C-O$ bond length in $CO$ is $1.128 \ \mathring{A}$)

The disease known as fluorosis is caused by the reaction of fluorine with which of the following?

The process requiring absorption of energy is

Which of the following is a favourable factor for cation formation?

$P$ is a point on the segment joining the feet of two vertical poles of heights $a$ and $b$. The angles of elevation of the tops of the poles from $P$ are $45^{\circ}$ each. Then,the square of the distance between the tops of the poles is

Three perfect gases at absolute temperatures $T_1, T_2$ and $T_3$ are mixed. The masses of molecules are $m_1, m_2$ and $m_3$ and the number of molecules are $n_1, n_2$ and $n_3$ respectively. Assuming no loss of energy,the final temperature of the mixture is