Determine the correctness or otherwise of the following Assertion [$a$] and the Reason [$r$] regarding mammalian cells.
Assertion [$a$]: Cells use $Ca^{2+}$, and not $Na^{+}$, for cell-to-cell signaling
Reason [$r$]: $n$ the cytosol, concentration of $Na^{+}$ is lower than that of $Ca^{2+}$
- Both [$a$] and [$r$] are true and [r] is the correct reason for [$a$].
- Both [$a$] and [$r$] are true but [$r$] is not the correct reason for [$a$].
- Both [$a$] and [$r$] are false.
- [$a$] is true but [$r$] is false.
