NEW YORK (AP) — U.S. gas prices fell below $4 a gallon on average Thursday, but just barely.

It is the first time since March that the average cost for a regular gallon has been that low. Prices fell overnight after President Donald Trump signed an agreement with Iran that calls for Tehran to dilute its stockpile of highly enriched uranium and waives U.S.-backed sanctions on the country.

Gas prices are at $3.999 on average in the U.S., according to the motor club AAA. The drop below $4 follows a 15% decline in the price of U.S. crude this month.

But fluctuations in gas prices remain across the country. In California, gas prices are averaging $5.64 per gallon, while in South Carolina, it’s $3.58 per gallon.

The agreement between the U.S. and Iran calls for a permanent end to hostilities and starts a 60-day negotiating clock to reach a final deal on the future of Iran’s nuclear program, though Trump left the door open to resume attacks. It appears to offer Iran several benefits up front while extracting little in return.

Oil prices fell Monday to about $80 for a barrel of U.S. benchmark crude. That compares to $67 per barrel before the war and the price of over $120 a barrel reached earlier in the conflict.