Solve this:

(a) The gas which has the largest quantity in the atmosphere: 
(i) oxygen       (ii) nitrogen     (iii) carbon dioxide      (iv) argon

(b) The layer of the crust which makes the ocean floor:
(i) sial         (ii) sima.         (iii) biosphere  (iv) lithosphere. 

(c) The number of continents on earth:
(i) five    (ii) seven      (iii) eight    (iv) six