Choose the correct set of international standards for drinking water. 

(P) ${\mathrm{F}}^{\ominus }<2 \mathrm{ppm}$ 

(Q) $\mathrm{Pb}<50 \mathrm{ppb}$

(R)${\mathrm{SO}}_4^{2-}\le 500 \mathrm{ppm}$

(S) ${\mathrm{NO}}_3^{-}\le 50\text{ ppm }$

Fluoride: Humans who are deficient in fluoride get tooth decay. In order to raise the concentration of soluble fluoride in the drinking water supply to 1 ppm, a technique known as water fluoridation is used.

Lead: Lead either exists in the water supply pipes, plumbing fixtures, soldiers, etc. Lead in excess can have specific neurological effects. Long-term exposure can endanger the health of unborn children and expectant mothers. It can also harm organs including the kidney, liver, etc. As a result, the only allowed lead concentration is 50 ppb.

Sulfate: In order to prevent algal growth in public water supply pipes, sulfate is frequently employed. Additionally, mines release them into the ocean. Some of the hydrates are very water-soluble. Therefore, they could still be present in water even after treatment. Although it is safe at a moderate dosage, if its concentration in water exceeds 500 ppm, it can lead to dehydration, diarrhea, specific laxative effects, and digestive issues.

Nitrate: Nitrates are used in fertilizers. They can reach surface and groundwater from agricultural fields. If the concentration of nitrates exceeds 50 ppm in water, the digestive system is affected. It also causes methemoglobinemia, commonly called a blue baby syndrome.