On what basis are the three types of lichens classified?

$A$ point $P$ moves in a counter-clockwise direction on a circular path as shown in the figure. The movement of $P$ is such that it sweeps out a length $S = t^3 + 5$,where $S$ is in meters and $t$ is in seconds. The radius of the path is $20 \, m$. The acceleration of $P$ when $t = 2 \, s$ is nearly ......... $m/s^2$.

If an electron in a hydrogen atom jumps from an orbit of level $n=3$ to an orbit of level $n=2$,then the emitted radiation frequency is (where $R=$ Rydberg's constant,$C=$ velocity of light).

Which of the following is not a class of Bryophytes?

In the following common emitter circuit,if $\beta = 100$,$V_{CE} = 7 \, V$,$V_{BE} = 0 \, V$ (negligible),and $R_C = 2 \, k\Omega$,then $I_B$ is.......$mA$.

Two particles of equal mass have velocities $\overrightarrow{v}_1=4 \hat{i} \text{ ms}^{-1}$ and $\overrightarrow{v}_2=4 \hat{j} \text{ ms}^{-1}$. The first particle has an acceleration $\overrightarrow{a}_1=(5 \hat{i}+5 \hat{j}) \text{ ms}^{-2}$ while the acceleration of the other particle is zero. The centre of mass of the two particles moves in a path of