Which of the following dopant is $NOT$ used in $Ge$ to obtain $n$-type semiconductor?

$ABC$ is an equilateral triangle. The length of each side is $a$ and the centroid is point $O$. Then $\overrightarrow{OA} + \overrightarrow{OB} + \overrightarrow{OC} = \dots$

What should be the angle $\theta$ between two vectors $\vec{A}$ and $\vec{B}$ so that the magnitude of the resultant vector $\vec{R}$ is maximum (in $^{\circ}$)?

Calculate the de Broglie wavelength of an electron in the first Bohr orbit of a hydrogen atom if the velocity of an electron in the first orbit is $2.2 \times 10^6 \text{ m s}^{-1}$. [mass of electron = $9.1 \times 10^{-31} \text{ kg}$, Planck's constant $(h)$ = $6.626 \times 10^{-34} \text{ J s}$]

Match Column $-I$ with Column $-II$.

Column $-I$	Column $-II$
$(1)$ Angular momentum	$(a)$ Scalar
$(2)$ Potential energy	$(b)$ Vector
	$(c)$ Unit vector

At what angle should the two forces $2P$ and $\sqrt{2}P$ act so that the resultant force is $P\sqrt{10}$ (in $^{\circ}$)?