Which two of the following are notable achievements of the maya? a. developing an extensive road system b. creating a hieroglyphic writing system c. inventing an accurate calendar system d. establishing a unified tribute empire e. creating advanced methods of irrigation

The two most notable achievements of the Maya (although an argument could be made that a few were accomplished) are B and C.

The Maya are considered the only precolonial American civilization to develop a complete writing system, which means they had a written language that could communicate any spoken idea. 

The Maya also developed an extremely accurate calendar, based on advanced knowledge of astronomy. You may remember the fuss about the Mayan calendar predicting the end of the world in December of 2012, because this is the time up to which that they had calculated dates and astronomical events for. Luckily, this didn't happen until 2016 (just kidding).