Which of the following alkali metals reacts directly with nitrogen to form a nitride?

An open pipe of length $33 \ cm$ resonates to a frequency of $1000 \ Hz$. The mode of vibration is: (velocity of sound $= 330 \ m/s$)

An auditorium has a volume of $10^5 \ m^3$ and a total absorption area of $2 \times 10^4 \ m^2$. Its average absorption coefficient is $0.2$. The reverberation time of the auditorium in seconds is:

The number of common tangents to the circles ${x^2 + y^2 - 4x - 6y - 12 = 0}$ and ${x^2 + y^2 + 6x + 18y + 26 = 0}$ is

Current is flowing through a conducting hollow pipe whose cross-section is shown in the figure. The value of magnetic induction will be zero at

The resultant gate and its Boolean expression in the given circuit is