Please read the following table in HTML format from a webpage and then answer the question in detail according to the table. Each table cell is defined by a <td> and a </td> tag. Each table row starts with a <tr> and ends with a </tr> tag. 'rowspan=m' and 'colspan=n' represents a cell spaning over 'm' rows and 'n' columns.

Webpage title:
German submarine U-751
Table title:
Summary of raiding history
Table:
<table id='hittask' contenteditable='False'>
<tr> <th colspan=1 rowspan=1 > Date </th><th colspan=1 rowspan=1 > Ship Name </th><th colspan=1 rowspan=1 > Nationality </th><th colspan=1 rowspan=1 > Tonnage </th><th colspan=1 rowspan=1 > Fate </th></tr>
<tr> <td colspan=1 rowspan=1 > 14 June 1941 </td><td colspan=1 rowspan=1 > St. Lindsay </td><td colspan=1 rowspan=1 > United Kingdom </td><td colspan=1 rowspan=1 > 5,370 </td><td colspan=1 rowspan=1 > Sunk </td></tr>
<tr> <td colspan=1 rowspan=1 > 21 December 1941 </td><td colspan=1 rowspan=1 > HMS Audacity </td><td colspan=1 rowspan=1 > Royal Navy </td><td colspan=1 rowspan=1 > 11,000 </td><td colspan=1 rowspan=1 > Sunk </td></tr>
<tr> <td colspan=1 rowspan=1 > 2 February 1942 </td><td colspan=1 rowspan=1 > Corilla </td><td colspan=1 rowspan=1 > Netherlands </td><td colspan=1 rowspan=1 > 8,096 </td><td colspan=1 rowspan=1 > Damaged </td></tr>
<tr> <td colspan=1 rowspan=1 > 4 February 1942 </td><td colspan=1 rowspan=1 > Silveray </td><td colspan=1 rowspan=1 > United Kingdom </td><td colspan=1 rowspan=1 > 4,535 </td><td colspan=1 rowspan=1 > Sunk </td></tr>
<tr> <td colspan=1 rowspan=1 > 7 February 1942 </td><td colspan=1 rowspan=1 > Empire Sun </td><td colspan=1 rowspan=1 > United Kingdom </td><td colspan=1 rowspan=1 > 6,952 </td><td colspan=1 rowspan=1 > Sunk </td></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > 16 May 1942 </td><td class="highlighted"  colspan=1 rowspan=1 > Nicarao </td><td class="highlighted"  colspan=1 rowspan=1 > United States </td><td class="highlighted"  colspan=1 rowspan=1 > 1,445 </td><td class="highlighted"  colspan=1 rowspan=1 > Sunk </td></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > 19 May 1942 </td><td class="highlighted"  colspan=1 rowspan=1 > Isabela </td><td class="highlighted"  colspan=1 rowspan=1 > United States </td><td class="highlighted"  colspan=1 rowspan=1 > 3,110 </td><td class="highlighted"  colspan=1 rowspan=1 > Sunk </td></tr>
</table>
Question:
What sank the ships Nicarao and Isabela?
Answer:
U-751 sank the American ships Nicarao and Isabela on 16 and 19 May 1942, totalling 1,455 and 3,110 GRTs respectively.