根据两个独立的小样本估计两个总体均值之差，当两个总体的方差未知且不相等时，使用的分布是（      ）。

A、<span style="FONT-FAMILY: 宋体; FONT-SIZE: 12pt; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-bidi-font-family: ">正态分布<span lang="EN-US"><span style="mso-spacerun: yes"></span></span></span>

B、<span style="FONT-FAMILY: ; FONT-SIZE: 12pt" lang="EN-US"><em>t</em></span><span style="FONT-FAMILY: 宋体; FONT-SIZE: 12pt; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-bidi-font-family: ">分布</span>

C、<span style="FONT-FAMILY: ; FONT-SIZE: 12pt" lang="EN-US"><em>χ</em></span>^{<span style="FONT-FAMILY: 宋体; FONT-SIZE: 12pt; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-bidi-font-family: " lang="EN-US">2</span>}<span style="FONT-FAMILY: 宋体; FONT-SIZE: 12pt; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-bidi-font-family: ">分布</span>

D、<span style="FONT-FAMILY: ; FONT-SIZE: 12pt" lang="EN-US"><em>F</em></span><span style="FONT-FAMILY: 宋体; FONT-SIZE: 12pt; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-bidi-font-family: ">分布</span>