In 2007, the population of north america was approximately 5.15 x 10^8 people and the population of south america was approximately 3.04 x 10^8 people. what is the combined population of north and south america for this year? a) 8.19 x 10^16 people b) 8.19 x 10^9 people c) 8.19 x 10^8 people d) 8.19 x 10^64 people