The person whos says: " they must have arrived by now".
a. is making a grammatical mistake
b. thinks it's probable that they have arrived
c. knows how long the journey takes
d. has some evidence for her prediction.

B. Thinks it's probable that they have arrived. 

Iwas in a argument about a certain class grade with my teacher. it was a ‘a-’when i felt that i deserved a ‘a’. i showed him (my teacher) why i felt i derserved so. i questioned what i got wrong and showed him what i learned. i look back to that teacher with nothing but love and support because that’s the class that i actually learned the most. the moral of the story is to see where the other person is coming from and what they have done. also, when in a conflict, never attack the persons character. this will just make the argument end poorly.