Both $A$ and $R$ true and $R$ is the correct explanation of $A .$
When a neuron is not conducting any impulse, i.e., resting, the axonal membrane is comparatively more permeable to potassium ions $\left(K^{+i i}\right)$ and nearly impermeable to sodium ions $\left(N a^{+i i}\right) .$ Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm. Consequently, the axoplasm inside the axon contains high concentration of $K^{+i i}$ and negatively charged proteins and low concentration of $N a^{+i i} .$ In contrast, the fluid outside the axon contains a low concentration of $K^{+i i}$, a high concentration of $N a^{+i i}$ and thus form a concentration gradient
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Reason : All life forms were in water environment only.