单选题A built-up exhaust valve is one in which （）.Athe stem and heads are made of different materialBlow-alloy steel is used throughoutCa replaceable valve disk is welded to the headDthe self-centering action comes from motion of the valve stem in the guide
单选题
A built-up exhaust valve is one in which （）.
A
the stem and heads are made of different material
B
low-alloy steel is used throughout
C
a replaceable valve disk is welded to the head
D
the self-centering action comes from motion of the valve stem in the guide
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