Highlight specific value of Listbox Dynamically C#

Question

In Windows Phone application, I have 3 values in Listbox

Low

Medium

High

 

Its XAML Code is
<ListBox HorizontalAlignment="Left" Height="103" VerticalAlignment="Top" Width="456" Name="priority" Margin="10,302,-10,0" Visibility="Visible">
<ListBoxItem x:Name="high" Content="High" Foreground="Black"/>
<ListBoxItem x:Name="medium" Content="Medium" Foreground="Black"/>
<ListBoxItem x:Name="low" Content="Low" Foreground="Black"/>
</ListBox>

 

When I click on button, I want to show "Low" value as Highlighted of Listbox (means dynamically), How I can write code for it in C# ?