Number of water molecules in one litre of pure water is

• A. \[\text{6}.0\text{23 }\times \text{ 1}{{0}^{\text{23}}}\]      
• B.        \[\text{3}.\text{45 }\times \text{ 1}{{0}^{\text{25}}}\]  
• C. \[\text{55}.\text{4 }\times \text{ 1}{{0}^{\text{23}}}\]       
• D.         \[\text{18 }\times \text{ 1}{{0}^{\text{23}}}\]