Which of the following is $NOT$ a characteristic of the phylum $Annelida$?

The energy of an electron in the $n^{th}$ orbit of a hydrogen atom is expressed as $E_n = \frac{-13.6}{n^2} \text{ eV}$. The shortest and longest wavelengths of the Lyman series are:

If the circle $x^2+y^2+6x-2y+k=0$ bisects the circumference of the circle $x^2+y^2+2x-6y-15=0$,then $k=$

The major product of the following reaction is:

If the acute angle between the lines $ax^2+2hxy+by^2=0$ is $\frac{\pi}{4}$,then $4h^2=$

If the distance between the centres of the Earth and the Moon is $D$ and the mass of the Earth is $81$ times the mass of the Moon,then at what distance from the centre of the Earth will the net gravitational field be zero?