As the polypeptide is elongating in translation, what is the ribosome doing?

As the polypeptide is elongating in translation,
  what is the ribosome doing?
 
  a. removing incorrectly added amino acids.
  b. moving along the mRNA transcript
  bonding amino acids to each other.
  c. travelling back and forth between the
  nucleus and the growing polypeptide with
  information on which amino acids to add.
  d. removing the non-coding introns.
  e. doing more than one of these functions

Question 2

Translation stops when
 
  a. enzymes attach to the mRNA at the end of
  the mRNA transcript.
  b. a certain number of codons have been read
  specified by the DNA gene.
  c. one of the three stop codons is encountered.
  d. the cell runs out of tRNA.
  e. more than one of these things happens