The correct statements with reference to single celled organisms are (i) complex substances are not broken down into simpler substances (ii) simple diffusion is sufficient to meet the requirement of exchange of gases (iii) specialised tissues perform different functions in the organism (iv) entire surface of the organism is in contact with the environment for taking in food. (a) (i) and (iii) (b) (ii) and (iii) (c) (ii) and (iv) (d) (i) and (iv).