When the North Vietnamese attacked Phuoc Long, they were testing the United States to see if the U. S. would . send the U. S.S. Maddox to the Gulf of Tonkin
invade Cambodia
send military advisors to the NLF
reenter the conflict

Reenter the conflict

Explanation:

When the North Vietnamese attacked Phuoc Long, they were testing the United States to see if the U.S. would reenter the conflict.

After the United States did not respond to the fall of Phuoc Long province, the North Vietnamese concluded that the United States would not reenter the conflict in Vietnam.

the andes or andean mountains (spanish: cordillera de los andes) are the longest continental mountain range in the world, forming a continuous highland along the western edge of south america. this range is about 7,000 km (4,300 mi) long, about 200 to 700 km (120 to 430 mi) wide (widest between 18° south and 20° south latitude), and of an average height of about 4,000 m (13,000 ft). the andes extend from north to south through seven south american countries: venezuela, colombia, ecuador, peru, bolivia, chile and argentina.

from wikipedia

hope this !

i believe the answer is d.