expert前加什么冠词

前面加冠词an，例如：

1、We employ an expert to advise on new technology.我们聘用了一位专家担任新技术顾问。

2、He earned a reputation as an expert on tax law.他赢得了税法专家的美名。

3、An expert with a trained eye will spot the difference immediately.训练有素、眼光敏锐的专家会马上发现差别所在。