What should the architect recommend to reduce data load processing time?

Universal Containers has a large volume of Contact data going into Salesforce.com. There are 100,000 existing contact records. 200,000 new contacts will be loaded. The Contact object has an external ID field that is unique and must be populated for all existing records.

What should the architect recommend to reduce data load processing time?
A . Load Contact records together using the Streaming API via the Upsert operation.
B . Delete all existing records, and then load all records together via the Insert operation.
C . Load all records via the Upsert operation to determine new records vs. existing records.
D . Load new records via the Insert operation and existing records via the Update operation.

Answer: D