Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Date | Tournament | Surface | Opponent | Score |
| --- | --- | --- | --- | --- |
| March 27, 2006 | Buenos Aires | Clay | Cristian Villagrán | 4–6, 6–3, 6–4 |
| April 24, 2006 | Buenos Aires | Clay | Leandro Migani | 6–3, 7–6 |
| August 7, 2006 | Córdoba | Clay | Leandro Migani | 5–7, 6–0, 6–0 |
| August 14, 2006 | Mendoza | Clay | Andrés Molteni | 6–4, 6–1 |
| September 11, 2006 | La Paz | Clay | Guillermo Carry | 6–3, 6–1 |
| September 18, 2006 | Cochabamba | Clay | Martín Alund | 7–6, 6–7, 7–6 |
| April 23, 2007 | Santa Fe | Clay | Juan-Martín Aranguren | 6–3, 6–2 |
| April 30, 2007 | Buenos Aires | Clay | Juan-Martín Aranguren | 6–4, 6–4 |
| May 7, 2007 | Tucuman | Clay | Juan Pablo Amado | 6–1, 6–4 |
| October 1, 2007 | Medellín | Clay | Chris Guccione | 7–5, 5–7, 7–5 |
| April 21, 2008 | Cremona | Hard | Björn Phau | 6–3, 6–4 |
| April 28, 2008 | Rome | Clay | Éric Prodon | 6–3, 6–7, 7–6 |
| May 12, 2008 | Bordeaux | Clay | Igor Kunitsyn | 6–2, 6–2 |
| October 25, 2009 | Santiago | Clay | Nicolás Massú | 6–2, 6–2 |
| November 22, 2009 | Lima | Clay | Jorge Aguilar | 7–5, 6–4 |
| January 31, 2010 | Bucaramanga | Clay | Juan Pablo Brzezicki | 6–4, 6–2 |
Question:
Who was the opponent on April 21, 2008?
Answer:
Björn Phau