Large crosshead type engine generally has two lubrication systems, a function performed only by cylinder oil system is（）

A.is coming upward；cylinder ring belt
B.is going downward；piston ring belt
C.is coming upward；piston ring belt
D.is going downward；cylinder ring belt

A.Large crosshead type；crossheads
B.High speed；the cylinders
C.Large crosshead type；the cylinders
D.Four-stroke；pistons and cylinders

A.removing heat and deposits
B.neutralizing acidic products
C.lubricating
D.improving combustion efficiency