<Tr> <Td> Rhododendron arborescens </Td> <Td> smooth azalea </Td> <Td> 4.2 </Td> <Td> 5.7 </Td> </Tr> <Tr> <Td> Pinus radiata </Td> <Td> Monterey pine </Td> <Td> 4.5 </Td> <Td> 5.2 </Td> </Tr> <Tr> <Td> Carya illinoinensis </Td> <Td> pecan </Td> <Td> 4.5 </Td> <Td> 7.5 </Td> </Tr> <Tr> <Td> Tamarindus indica </Td> <Td> tamarind </Td> <Td> 4.5 </Td> <Td> 8.0 </Td> </Tr>

Which of the following is generally considered the source of acid rain