<Tr> <Td> Si </Td> <Td> 27.7 </Td> </Tr> <Tr> <Td> Al </Td> <Td> 8.1 </Td> </Tr> <Tr> <Td> Fe </Td> <Td> 5.0 </Td> </Tr> <Tr> <Td> Ca </Td> <Td> 3.6 </Td> </Tr>

Which of the following is true of earth’s crust