Which of the reasons below best explains why america was unable to defeat the north vietnamese?

a
americans were unable to fight in the jungle since they weren't used to the climate or conditions. ultimately, they were too sick and tired to defeat the enemy.
b
the vietnamese people proved to be far more committed to winning the war and defending their country than the americans were to defeating communism.
c
americans went into vietnam without support from the un. they were unable to win the war without support from their allies.
d
north vietnam had much more sophisticated technology than the us did, supplied by their communist supporters in the ussr.