(a) Wheat: The recommended seed rate is 100-125 kg per hectare under normal conditions, depending on the variety and planting method.
(b) Mustard: The recommended seed rate is 4-5 kg per hectare, as mustard requires lower seed density for proper spacing.
These seed rates ensure optimal plant population and maximum yield.
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: $S_n = \frac{n}{2} \left[ a + (n - 1) d \right].$