若墙背光滑，则由库仑土压理论计算出的主动土压力 ，其作用方向(  )

A、<span style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-bidi-font-family: " times="" new="" roman";="" mso-font-kerning:="" 1.0pt;="" mso-ansi-language:="" en-us;="" mso-fareast-language:="" zh-cn;="" mso-bidi-language:="" ar-sa"="">必为水平的<span lang="EN-US"><span style="mso-spacerun: yes"> </span></span></span>

B、<span style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-bidi-font-family: " times="" new="" roman";="" mso-font-kerning:="" 1.0pt;="" mso-ansi-language:="" en-us;="" mso-fareast-language:="" zh-cn;="" mso-bidi-language:="" ar-sa"="">必为铅垂的</span>

C、<span style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-bidi-font-family: " times="" new="" roman";="" mso-font-kerning:="" 1.0pt;="" mso-ansi-language:="" en-us;="" mso-fareast-language:="" zh-cn;="" mso-bidi-language:="" ar-sa"="">必垂直于墙背 </span>

D、<span style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-bidi-font-family: " times="" new="" roman";="" mso-font-kerning:="" 1.0pt;="" mso-ansi-language:="" en-us;="" mso-fareast-language:="" zh-cn;="" mso-bidi-language:="" ar-sa"="">不一定</span>