The structure of $IF_7$ is $......$ .

Which of the following compounds contains an $S - S$ bond?

Match the columns.

Column $-I$	Column $-II$
$(a)$ Parturition	$(p)$ Attachment of embryo to endometrium
$(b)$ Gestation	$(q)$ Release of egg from Graafian follicle
$(c)$ Ovulation	$(r)$ Delivery of Foetus from uterus
$(d)$ Implantation	$(s)$ Duration between pregnancy and birth
	$(t)$ Formation of zygote by fusion of egg and sperm

Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combination is

Check whether the following pair of statements are negations of each other. Give reasons for your answer.
$I$: $x+y=y+x$ is true for every real number $x$ and $y$.
$II$: There exists real numbers $x$ and $y$ for which $x+y \neq y+x$.

Which of the following has the lowest thermal stability?