If the excess pressure inside a soap bubble is balanced by an oil column of height $2\; mm$,then the surface tension of the soap solution will be ($r = 1\; cm$ and density $d = 0.8\; g/cm^3$).

The correct sequence of plants in a hydrosere is

In the following sequence of reactions,the alkene affords the compound $B$:
$CH_3-CH=CH-CH_3$ $\xrightarrow{O_3} A$ $\xrightarrow{Zn, H_2O} B$
The compound $B$ is

Three rods $AB$,$BC$,and $BD$ made of the same material and having the same cross-section have been joined as shown in the figure. The ends $A$,$C$,and $D$ are held at temperatures of $20^{\circ}C$,$80^{\circ}C$,and $80^{\circ}C$ respectively. If each rod is of the same length,then the temperature at the junction $B$ of the three rods is: (in $^{\circ}C$)

If a gas expands at constant temperature,it indicates that

Which group of plants was the first to exist on Earth?