There are two people rock climbing, one is on the ground while the other climbs.
People climbing rocks.
Its referred to people because it's more than one person.
exists p1 x1 x6 x7.(-(x6 = x3) & n_climb(x6) & n_ground(x7) & exists p2 x2.((x1 = x2) & exists p3 x3 x4 x5.(while(p3,x6) & n_climb(x3) & n_climb(x3) & on(x3,x7) & rel(x3,x5) & n_thing(x5) & card_eq_1(x5) & of(x3,x4) & n_rock(x4)) & that(x2,p2) & n_people(x2) & card_eq_2(x2)))
exists e1 x1 x2 x3.(a_topic(x1) & Theme(e1,x3) & Actor(e1,x2) & v_climb(e1) & n_rock(x3) & (x1 = x2) & n_people(x1))