Select the correct answer. Which of these physical concepts is most important in civil engineering?
A. energy transformations
B. motion under gravity
C. heat transfer
D. forces in structures
E. atomic structure

Acircular pipe of 30mm outside diameter is placed in an airstream at 30 c and latm pressure. the air moves in cross flow over the pipe at 20 m/s, while the outer surface of the pipe is maintained at 110 c. what is the drag force exerted on the pipe per unit length? what is the rate of heat transfer from the pipe per unit length?