Which disease in tobacco was identified by $D.J. Ivanowsky$ as being caused by viruses?

$A$ magnetic needle lying parallel to a magnetic field requires $W$ units of work to turn it through $60^{\circ}$. The torque required to maintain the needle in this position will be

Which of the following is not a unit of time?

In any $\triangle ABC$,the expression $\frac{(a+b+c)(b+c-a)(c+a-b)(a+b-c)}{4b^2c^2}$ is equal to:

Synapsis occurs between

$A$ Van de Graaff generator has a spherical metal shell as an electrode which is at a potential $15 \times 10^6 \ V$. If the dielectric strength of the surrounding medium is $5 \times 10^7 \ V \ m^{-1}$,then the diameter of the shell is: (in $cm$)