An unknown nucleus has a nuclear density of $2.29 \times 10^{17} \mathrm{~kg} / \mathrm{m}^3$ and mass of $19.926 \times 10^{-27} \mathrm{~kg}$. Its mass number $A$ is approximately:

(Take $R_0=1.2 \times 10^{-15} \mathrm{~m}, 4 \pi=12.56$ )

A particle of mass $m$ is moving around the origin with a constant force $F$ pulling it towards the origin. If Bohr model is used to describe its motion, the radius of the $n^{\text {th }}$ orbit and the particle's speed $v$ in the orbit depend on $n$ as

The spectral series which corresponds to the electronic transition from the levels $$n_2=5,6, \ldots$$ to the level $$n_1=4$$ is

Water is used as a coolant in a nuclear reactor because of its