AJAX方法调用未定义- ASP.NET C#中未加载GoogleCharts

<script type="text/javascript" src="https://www.gstatic.com/charts/loader.js"></script>

google.charts.load('current', { 'packages': ['corechart'] });
google.charts.setOnLoadCallback(drawChart);

function drawChart() {
  // 创建数据表
  var data = new google.visualization.DataTable();
  data.addColumn('string', 'Task');
  data.addColumn('number', 'Hours');
  data.addRows([
    ['Work', 8],
    ['Eat', 2],
    ['Sleep', 6],
    ['Play', 4],
    ['Study', 5]
  ]);

  // 设置图表选项
  var options = {
    title: 'My Daily Activities',
    pieHole: 0.4
  };

  // 创建图表对象并绘制图表
  var chart = new google.visualization.PieChart(document.getElementById('chart_div'));
  chart.draw(data, options);
}

<div id="chart_div"></div>

protected void Page_Load(object sender, EventArgs e)
{
    ScriptManager.RegisterStartupScript(this, GetType(), "drawChart", "drawChart();", true);
}