useful tool前用a还是用an

前面用a，例如：

1.The cellphone is a useful tool.手机是很有用的工具。

2.Pepper spray can be a useful tool.辣椒喷雾是个挺有用的工具。

3.That would have been a useful tool 15 years ago.这本该是15年前一种得力的思考方式。

4.The takeaway from these two studies is that urban green space may help to deter crime and could be a useful tool in city-wide crime prevention policies.

用an useful是以元音音素开头的单词。