Save checked ASP.Net CheckBox value in Database using C#

<div runat="server" id="addquestion" style="border-style: solid; border-width: 5px;"
    visible="false">
    <div class="has-feedback col-sm-12">
        <label class="control-label">
            Question</label>
        <asp:TextBox ID="txtquestion" runat="server" required="required" class="form-control"
            placeholder="Enter The Question"></asp:TextBox>
    </div>
    <div class="has-feedback col-sm-12">
        <label class="control-label">
            Answer</label>
        <asp:TextBox ID="txtanswer" runat="server" required="required" class="form-control"
            placeholder="Enter The Answer "></asp:TextBox>
    </div>
    <div class="has-feedback col-sm-12 row">
        <div class="col-sm-3">
            <label class="control-label">
                Easy</label>
            <asp:CheckBox ID="checkbox1" runat="server" required="required" class="form-control"
                Text=" First Level"></asp:CheckBox>
        </div>
        <div class="col-sm-3">
            <label class="control-label">
                Inter Medium</label>
            <asp:CheckBox ID="checkbox2" runat="server" required="required" class="form-control"
                Text="Second Level"></asp:CheckBox>
        </div>
        <div class="col-sm-3">
            <label class="control-label">
                Advance</label>
            <asp:CheckBox ID="checkbox3" runat="server" required="required" class="form-control"
                Text="Third Level"></asp:CheckBox>
        </div>
    </div>
    <div class="col-sm-12">
        <asp:Button ID="Submitquestion" Style="background-color: midnightblue; border-radius: 3px;"
            OnClick="Submitquestion_Click" runat="server" class="btn btn-group btn-default btn-sm"
            Text="Submit" />
    </div>
</div>

protected void Submitquestion_Click(object sender, EventArgs e)
{
    obj.CreateConnectionsql(); 
    string query = "insert into  Question(Question , Answer,  Chapter_name, Question_type_id,Level ) values('" + txtquestion.Text + "','" + txtanswer.Text + "','" + ddlchaptername.SelectedIndex + "','" + ddlquestionlevel.SelectedIndex + "','" + checkbox1.Checked + "," + checkbox2.Checked + "," + checkbox3.Checked + "')";
    SqlCommand cmd = new SqlCommand(query, clsDataAccess.Conn); 
    cmd.ExecuteNonQuery(); 
    txtquestion.Text = string.Empty;
    txtanswer.Text = string.Empty; 
    checkbox1.Text = string.Empty; 
    checkbox2.Text = string.Empty; 
    checkbox3.Text = string.Empty; 
    obj.CloseConnection();
}