当正态总体方差未知时，样本均值经过标准化后服从的分布为（ ）。

A、<IMG src="https://s3.cn-north-1.amazonaws.com.cn/qingshuxuetang/staticQuestionImg/d26b4e08-77ca-449f-b16e-c5c8cc6f88ad.gif"title="ex_1257339030379_0.gif" alt="ex_1257339030379_0.gif"/>

B、<IMG src="https://s3.cn-north-1.amazonaws.com.cn/qingshuxuetang/staticQuestionImg/62ac531a-c6cb-4d02-977f-9912fb173f8d.gif"title="ex_1257339041787_0.gif" alt="ex_1257339041787_0.gif"/>

C、<IMG src="https://s3.cn-north-1.amazonaws.com.cn/qingshuxuetang/staticQuestionImg/34115177-898b-46bb-9da2-974c9e4827ae.gif"title="ex_1257339056530_0.gif" alt="ex_1257339056530_0.gif"/>

D、<IMG src="https://s3.cn-north-1.amazonaws.com.cn/qingshuxuetang/staticQuestionImg/073df1c6-8557-4c14-b436-55d718d91012.gif"title="ex_1257339068573_0.gif" alt="ex_1257339068573_0.gif"/>