Which of the following could act as a propellant for rockets?

Two spherical soap bubbles of radii $r_1$ and $r_2$ in vacuum combine under isothermal conditions. The resulting bubble has a radius equal to:

In the given figure,what do $A$ and $B$ represent respectively?

Let $PQR$ be a right-angled isosceles triangle,with the right angle at $P(2, 1)$. If the equation of the line $QR$ is $2x + y = 3$,then the equation representing the pair of lines $PQ$ and $PR$ is

The position of a particle is given by $\overrightarrow {r} = (\hat i + 2\hat j - \hat k)$ and its momentum is given by $\overrightarrow {P} = (3\hat i + 4\hat j - 2\hat k)$. The angular momentum is perpendicular to:

The quantum numbers $+1/2$ and $-1/2$ for the electron spin represent