Who discovered viroids?

What is the percentage of Photosynthetically Active Radiation $(PAR)$ in the incident solar radiation?

The packing efficiency of the two-dimensional square unit cell shown below is ........... $ \% $

$A$ long straight wire,of radius $a$,carries a current distributed uniformly over its cross-section. The ratio of the magnetic fields due to the wire,at distance $\frac{a}{3}$ and $2a$ respectively from the axis of the wire,is

Observe the following reaction: $2A + B \longrightarrow C$. The rate of formation of $C$ is $2.2 \times 10^{-3} \ mol \ L^{-1} \ min^{-1}$. What is the value of $-\frac{d[A]}{dt}$ (in $mol \ L^{-1} \ min^{-1}$)?

$A$ cubical block of wood with a side length of $10 \, cm$ floats at the interface between oil and water,with its lower surface horizontal and $4 \, cm$ below the interface. The density of oil is $0.6 \, g \cdot cm^{-3}$ and the density of water is $1 \, g \cdot cm^{-3}$. The mass of the block is ...... $gm$.