Question

____ the students at the library last month? a) were b) was c) are d) is

Answer 1

La respuesta es c 
ya que diria:
Are the students at the library last month? 
Yes, The students at the library last month

Answer 2

la respuesta correcta es WERE the students at the library last month?
fíjate que estamos hablando en PASADO, por lo tanto es WERE.