Before the discovery of oil, frankincense was Oman’s main source of wealth. As valuable as gold, it was revered throughout the ancient world, and fortunes were made shipping the resin throughout the Roman Empire. Although it is no longer as valued as gold, frankincense is still, in its own way, invaluable in Oman—as history and as culture. According to the passage, Oman's only export that is more valuable than frankincense is which of the following?
The passage states that until the discovery of oil, Oman's most valuable commodity was frankincense.