如何解决“类型或命名空间名称'Systems‘在命名空间’Fizzler‘中不存在”？

Question

因此，我使用Nuget获得了Fizzler包，它关闭了HTML敏捷包，但现在我在尝试使用该库时遇到了编译错误

The type or namespace name 'Systems' does not exist in the namespace 'Fizzler'

package.config包含

 <?xml version="1.0" encoding="utf-8"?>
 <packages>
    <package id="Fizzler" version="1.0.0" targetFramework="net45" />
    <package id="HtmlAgilityPack" version="1.4.9.5" targetFramework="net45" />
 </packages>

我试图调用HTML5 CSS3选择器方法QuerySelectorAll，但它给出了错误消息... error CS1061: 'HtmlAgilityPack.HtmlNode' does not contain a definition for 'QuerySelectorAll' and no extension method 'QuerySelectorAll' accepting a first argument of type 'HtmlAgilityPack.HtmlNode' could be found (are you missing a using directive or an assembly reference?)