Where is the c4 at the bottom of strut a in Metal Gear Solid 2

It is located underneath the submersible in the very first area you arrive in on the plant. Go to the edge of the railing adjacent to the stairs leading to the lockers. Press against the railing and press triangle to leap over the side. Shimmy your way right past the fence and press triangle again to leap over to the other side of the railing. Now just walk slowly up the steps (to avoid falling off), crouch, and spray the C4 underneath.