M点高程HM=43.251m，测得后视读数a=000m，前视读数b=2.283m。则视线高Hi和待求点N的高程HN分别为（　　）。

A.45.534m；43.251m

B.40.968m；38.685m

C.44.251m；41.968m

D.42.251m；39.968m