CRI (Crown Root Initiation) is the most critical stage of irrigation for wheat. Adequate water supply at this stage ensures proper root development, which is vital for nutrient and water uptake.
Assertion (A): The sum of the first fifteen terms of the AP $21, 18, 15, 12, \dots$ is zero.
Reason (R): The sum of the first $n$ terms of an AP with first term $a$ and common difference $d$ is given by: $S_n = \frac{n}{2} \left[ a + (n - 1) d \right].$