http://www.youtube.com/watch?NR=1&v=8yitJYxFixk&feature=endscreen 9:24 one step forward one step back 一進一退
http://www.youtube.com/watch?v=-Cw-K3fsr-E&list=FLcHEbpS1ymoTEBw7IRBh2hA&feature=mh_lolz 41:29 one step forward two steps back
http://www.youtube.com/watch?v=DM29Kw5RR6o 2:03 so they take us two steps forward one step back
http://www.youtube.com/watch?v=8ddGrM2QVnM 2:04 Taking one step forward slipping two steps back
練習問題解答
a-1377 one step forward and two steps back
a-1900 two steps forward, one step back