Which one of the following is a correct statement

• A. the exhaust valve of a four stoke I.C. engine should open exactly at the end of expansion stroke
• B. in actual practice, inlet valve of a four stroke I.C. engine should open exactly at the start of suction stroke
• C. the inlet valve remains open approximately for 180?
• D. valve overlap is the angle during which both the inlet and exhaust valves remain open simultaneously
• E. None of the above