Which of the following substances is used in the laboratory to dry natural gas quickly?

If the pair of straight lines $xy-x-y+1=0$ and the line $ax+2y-3a=0$ are concurrent,then $a$ is equal to

The resistance of a bulb filament is $100\, \Omega$ at a temperature of $100\, ^oC$. If its temperature coefficient of resistance is $0.005\, (^oC)^{-1}$,at what temperature will its resistance become $200\, \Omega$?

In how many ways can a team of $10$ players be selected from $22$ players,such that $6$ particular players are always included and $4$ particular players are always excluded?

The equation of the pair of lines passing through the point $(2, 1)$ and perpendicular to the pair of lines $4xy + 2x + 6y + 3 = 0$ is:

The principle of immunization or vaccination is based on the property of ...........