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.