Which of the following is a $\pi$-acid ligand?

$A$ marble block of mass $2\, kg$ lying on ice when given a velocity of $6\, m/s$ is stopped by friction in $10\, s$. Then the coefficient of friction is-

The major product obtained on interaction of phenol with sodium hydroxide and carbon dioxide is

Two identical short bar magnets,each having magnetic moment $M$,are placed a distance of $2d$ apart with their axes perpendicular to each other in a horizontal plane. The magnetic induction at a point midway between them is:

$A$ crystal of intrinsic silicon at room temperature has a carrier concentration of $1.6 \times 10^{16} / m^3$. If the donor concentration level is $4.8 \times 10^{20} / m^3$,then the concentration of holes in the semiconductor is

What will be the phase difference between virtual voltage and virtual current,when the current in the circuit is wattless (in $^o$)?