Which one of the following is commonly used in the transfer of foreign $DNA$ into crop plants?

In a $p-n$ junction diode,the thickness of the depletion layer is $2 \times 10^{-6} ~m$ and the barrier potential is $0.3 ~V$. The intensity of the electric field at the junction is:

From a disc of radius $R$ and mass $M$,a circular hole of diameter $R$,whose rim passes through the centre is cut. What is the moment of inertia of the remaining part of the disc about a perpendicular axis,passing through the centre?

The contrapositive of the statement "If two numbers are not equal,then their squares are not equal" is

Changes in $GnRH$ pulse frequency in females is controlled by circulating levels of

$A$ player tosses $2$ fair coins. He wins Rs. $5$ if $2$ heads appear,Rs. $2$ if $1$ head appears,and Rs. $1$ if no head appears. The variance of his winning amount is: