Fill in the blanks with correct preposition with the help of given options:

The FBI field office is a four storey brick fortress (a)_________(at, on, towards, for) the southern shore of 3 lake pontchartrain. We stop at the heavy iron gate (b)__________ (that, for, as, to) show an armed guard our credentials. Once inside, we hurry (c)__________(through, in, by, outside) to the vestibule where a woman awaits us.