In a four speed four-stroke Diesel engine, the intake valve

• A. opens at T.D.C. and closes at B.D.C.
• B. opens at 20 before T.D.C. and closes at 35 after B.D.C.
• C. opens at 10 after T.D.C. and closes at 20 before B.D.C.
• D. none of the above