WPF，GroupBox。无法将第二个按钮添加到GroupBox

Question

只是，试图将第二个按钮添加到GroupBox上，毕竟知道它是 Group Box，并且感到惊讶，这是WPF不允许的。我说的对吗？

<Window x:Class="WpfApplication2.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:WpfApplication2"
    mc:Ignorable="d"
    Title="MainWindow" Height="768" Width="1024"  MinHeight="768" MinWidth="1024" MaxHeight="1080" MaxWidth="1920">

<Grid x:Name="Grid_1"  ShowGridLines="True">
    <Grid.RowDefinitions>
        <RowDefinition Height="80*"/>
        <RowDefinition Height="512*"/>
        <RowDefinition Height="160*"/>
    </Grid.RowDefinitions>
    <Image x:Name="image"  Grid.Row="1" Source="Resources/truck.png" />
    <Grid  Grid.Row="2">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="112*"/>
            <ColumnDefinition Width="247*"/>
            <ColumnDefinition Width="247*"/>
            <ColumnDefinition Width="247*"/>
            <ColumnDefinition Width="118*"/>
        </Grid.ColumnDefinitions>
        <GroupBox x:Name="groupBox1" Header="Truck1" FontSize="16" Background="BlanchedAlmond" Grid.Column="1" >
            <Button x:Name="button" Content="Start1" HorizontalAlignment="Left" Margin="154,6,0,0" VerticalAlignment="Top" Width="75"/>
        </GroupBox>
        <GroupBox x:Name="groupBox2" Grid.Column="2" Header="Truck2" FontSize="16" Background="BlanchedAlmond"   />
        <GroupBox x:Name="groupBox3" Grid.Column="3" Header="Truck3" FontSize="16" Background="BlanchedAlmond"   />
    </Grid>





</Grid>