在下列一段话中的横线上填入下列语句，衔接最恰当的一项是（3分）一条石阶小路折转而上， 。树并不很粗，却坚韧挺拔，都生长在石上。 。树冠顶部都挑着几根青青的疏枝，在风中摇曳着。

One advantage of playing the guitar is _________ it can give you a great deal of pleasure.

There are plenty of jobs _____ in the western part of the country. [ ]

You promised your friend a letter; you ought to ______ days ago.

The teacher encourages the children to feel about asking questions.