CSS - select th tags only within the tbody of a table

Question

I would like to use a css selector to get only the th tags with the tbody. There are also th tags in the thead section, which I don't want included by the selector. Here's the markup I'm working with. Is there a selector to accomplish this?

<table class="bgtable">
<thead><tr><td width="40%">&nbsp;</td>
<th class="tdplain">Grade 4</th>
<th class="tdplain">Grade 8</th>
<th class="tdplain">Grade 12</th>
</tr>
</thead>
<tbody><tr><th class="tdplain">Civics (2010)</th>
<td class="tdplain">769K</td>
<td class="tdplain">577K</td>
<td class="tdplain">1179K</td>
</tr>
</tbody>
</table>

Answer 1

.bgtable tbody th {
   color: red;
}

Answer 2

table.bgtable tbody th {
 /* CSS rules here */
}

Answer 3

tbody>tr>th {color:red;}