The hardest natural substance known is: MCQ with Answer and Explanation

Explanation:
DAP, (NH₄)₂HPO₄, supplies nitrogen (18% N) and phosphorus (46% P₂O₅). It is a widely used phosphorus-rich fertilizer. Potassium is not present. NPK complex fertilizers provide all three. DAP is highly water-soluble, making phosphorus readily available. In soil, DAP temporarily increases pH around granules due to ammonia, but later nitrification lowers pH. Application: usually at sowing time.

Explanation:
Periods are horizontal rows (7 periods). Groups are vertical columns (18). Elements in same period have same number of electron shells.

Explanation:
Iodine-131 is a radioactive isotope used to treat thyroid disorders including goitre and cancer. Carbon-14 for dating, Cobalt-60 for cancer therapy, Uranium-235 as fuel in nuclear reactors. Non-radioactive isotopes are used as tracers. Goitre is often caused by iodine deficiency, but radioactive iodine can be used therapeutically.