a air还是 an air

是an air，例如：

1.She assumed an air of confidence in spite of her dismay.她尽管惊慌，但仍装作有信心的样子。

2.She had an air of restrained fury.她那样子似乎心里有压抑着的怒火。

3.There was an air of expectancy among the waiting crowd.等待的人群有一种期盼的心情。