Which of the following animals has a myogenic heart?

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

What is $X$ in the following reaction?
$2 CH_3COCH_3 \stackrel{Ba(OH)_2}{\longrightarrow} X$

The increasing order of stability of the following free radicals is:

Seven white balls and three black balls are randomly arranged in a row. The probability that no two black balls are placed adjacently is

The average marks of $10$ students in a class was $60$ with a standard deviation of $4$,while the average marks of another $10$ students was $40$ with a standard deviation of $6$. If all the $20$ students are taken together,their standard deviation will be: