$NPN$ transistors are preferred to $PNP$ transistors because they have

$NH_3 + O_2 \xrightarrow[\Delta]{Pt} A + H_2O;$
$A + O_2 \to B$
$B + O_2 + H_2O \to C$
$A, B$ and $C$ are:

Which has the maximum number of molecules among the following?

Three non-zero non-collinear vectors $\vec{a}, \vec{b}$ and $\vec{c}$ are such that $\vec{a}+3\vec{b}$ is collinear with $\vec{c}$,and $3\vec{b}+2\vec{c}$ is collinear with $\vec{a}$. Then $\vec{a}+3\vec{b}+2\vec{c}$ equals to

Parts $A, B, C$ and $D$ of the human eye are shown in the diagram. Select the option which gives correct identification along with its functions / characteristics:

What is the period of small oscillations of the block of mass $m$ if the springs are ideal and pulleys are massless?