The average temperature of the first three days of a week is \[{{27}^{o}}C\] and that of the next three days is\[{{29}^{o}}C\]. If the weekly average is\[{{28.5}^{o}}C\], what is the temperature on the last day?

• A. \[{{31.5}^{o}}C\] 
• B. \[{{28}^{o}}C\] 
• C.        \[{{21}^{o}}C\] 
• D.        \[{{42}^{o}}C\]