Two blocks are connected by a spring. The combination is suspended,at rest,from a string attached to the ceiling,as shown in the figure. The string breaks suddenly. Immediately after the string breaks,what is the initial downward acceleration of the upper block of mass $2\,m$?

The enzyme involved in the transfer of electrons is .......

In which of the following does aluminum become passive?

Which of the following compounds can form a zwitterion?

Which of the following represents soap?

$A$ parallel plate capacitor is charged to a potential difference of $100\ V$ and disconnected from the source of emf. $A$ slab of dielectric is then inserted between the plates. Which of the following three quantities change?
$(i)$ The potential difference
$(ii)$ The capacitance
$(iii)$ The charge on the plates