题目

They sat down opposite each other, but it was some moments ______ they spoke. A. after   B. before C. since   D. when 答案：B 如下图所示，如果茎a侧生长素在B点以下的浓度范围内。下列对b侧生长素浓度范围的描述哪项较为准确(　    ) A．在OA段范围内      B．在BC段范围内 C．在BD段范围内      D．在BA段范围内