pH of water is $7.0$ at $25^{\circ} \mathrm{C}$. If water is heated to $70^{\circ} \mathrm{C}$, the
(a) $\mathrm{pH}$ will decrease and the sample becomes acidic.
(b) $\mathrm{pH}$ will increase but the sample will remain neutral.
(c) pH will remain constant as 7 .
(d) pH will decrease but the sample will remain neutral.