In the flexural performance experiment conducted on a under-reinforced beam, which is the correct description of the change of the neutral axis. ( )

A、Thepositionoftheneutralaxisremainsunchanged.;
B、Thepositionoftheneutralaxisisconstantlymovingup.;
C、Thepositionoftheneutralaxisisconstantlymovingdown.;
D、Thepositionoftheneutralaxismovesupbeforeyieldingofthebeamandthenshiftsdownafteryieldingofthebeam.