Which of the following is the correct electronic configuration of $Cr$ $(Z=24)$?

$A$ closed pipe is suddenly opened and changed to an open pipe of the same length. The fundamental frequency of the resulting open pipe is less than that of the $3^{rd}$ harmonic of the earlier closed pipe by $55 ~Hz$. Then,the value of the fundamental frequency of the closed pipe is: (in $~Hz$)

Identify $A$ and $B$ in the following reaction:
$A \xrightarrow[\Delta]{aq. NaOH} C_2H_5OH \xleftarrow{AgOH} B$

In mammals,histamine is secreted by .........

If the sum of any two roots of the equation $x^3+p x^2+q x+r=0$ is zero,then

Which of the following compounds will react with $NaHCO_3$ solution to give sodium salt and carbon dioxide?