The pH of several solutions was measured in the research laboratories of a food company; convert each of the following pH values into the molar concentration of H₃O⁺ ions: 

(a) 3.3 (the pH of sour orange juice); 

(b) 6.7 (the pH of a saliva sample); 

(c) 4.4 (the pH of beer); 

(d) 5.3 (the pH of a coffee sample).