Find the missing letter:
Find $P_1 + P_2 + \dots + P_{10}$ if $P_k$ is the perimeter of a square having side length $k$.
| Column I | Column II |
|---|---|
| (a) Lecture Method | (II) Teacher-centered, focuses on delivery |
| (b) Demonstration Method | (I) Learning by doing |
| (c) Project Method | (III) Students explore real-life problems |
| (d) Heuristic Method | (IV) Learning through discovery |