Read the passage.

In 1856, the government was taking steps to create a public-school system. Progressing, the legislature passed the first public-school law in 1860. This encouraged other districts that were building schools in other parts of the territory. The University of New Mexico was built for the increasing population of Albuquerque in 1889. Established schools became more numerous in some areas by the early 1900s.

Which underlined word is a past participle?

a. Established
b. building
c. Progressing
d. passed