Which kind of air mass comes from canada?

a) continental tropical

b) maritime tropical

c) maritime polar

d) continental polar

The answer is option D.) Continental Polar. 

Six amino acids have side chains that are polar but not charged. these are serine (ser), threonine (thr), cysteine (cys), asparagine (asn), glutamine (gln), and tyrosine (tyr). these amino acids are usually found at the surface of proteins, as discussed in the proteins 2 module.