circulate的名詞形式

　　例句：

　　Draughts help to circulate air.

　　通風有助于循環空氣。

　　Leave a gap at the top and bottom so air can circulate.

　　在頂部和底部各留一點空隙，這樣空氣就可以流通了。

　　Rumours began to circulate about his financial problems.

　　有關他財務困難的謠言開始流傳開來。