1. Seed: A seed is a matured ovule containing an embryo, used for planting and producing new plants. It is specifically processed and treated for sowing purposes.
2. Grain: A grain is the harvested product of cereal crops, used primarily for consumption or trade. It is not treated or processed for planting. Key Difference: Seeds are selected for reproduction, while grains are for consumption.
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: $S_n = \frac{n}{2} \left[ a + (n - 1) d \right].$