<Tr> <Td> Week 1 </Td> <Td> none </Td> <Td> BRD Bucharest Open (Bucharest, Romania) Swedish Open (Båstad, Sweden) </Td> </Tr> <Tr> <Td> Week 2 </Td> <Td> Swedish Open (Båstad, Sweden) Croatia Open (Umag, Croatia) </Td> <Td> Gastein Ladies (Bad Gastein, Austria) </Td> </Tr> <Tr> <Td> Week 3 </Td> <Td> German Open Tennis Championships (Hamburg, Germany) Swiss Open (Gstaad, Switzerland) </Td> <Td> none </Td> </Tr> <Tr> <Td> Week 4 </Td> <Td> Austrian Open Kitzbühel (Kitzbühel, Austria) </Td> <Td> none </Td> </Tr>

What are the white lines made of on a clay tennis court