How many nodal tissues (specialized cardiac musculature) are present in the human heart?

When a vibrating tuning fork is placed on a sound box of a sonometer, $8$ beats per second are heard when the length of the sonometer wire is kept at $101 \, cm$ or $100 \, cm$. Then the frequency of the tuning fork is (Consider that the tension in the wire is kept constant). (in $ \, Hz$)

The minimum value of $27 \tan ^2 \theta + 3 \cot ^2 \theta$ is

Teak wood is obtained from which plant?

In non-cyclic photophosphorylation,the electron released by $PSI$ is accepted by ..........

$A$ thin prism $P_1$ with angle $4^o$ and made from glass of refractive index $1.54$ is combined with another thin prism $P_2$ made from glass of refractive index $1.72$ to produce dispersion without deviation. The angle of prism $P_2$ is (in $^o$)