去括号，合并同类项：3x+2（y﹣x）﹣（﹣x﹣4y）．
解：原式=3x+2y﹣2x+x+4y
=2x+6y．
催化剂的选择性
·Look at the questions for this part. ·You will hear a passage about "American Weighs In ". You will listen to it twice. ·For questions 24-30, indicate which of the alternatives A, B, or C is the most appropriate response. ·Mark one letter A, B, or C on the Answer Sheet.
Koop once launched an assault on ______.
A．chronic ailments
B．obesity
C．smoking
