The ratio of two unequal sides of a rectangle is 1 : 2. If its perimeter is 24 cm, then the length of diagonal in cm is:

• A. $$ {2 over { sqrt {5}}}$$
• B. $$ {4 over { sqrt {5}}}$$
• C. $$ {2 { sqrt {5}}}$$
• D. $$ {4 { sqrt {5}}}$$