Which sentence contains a misplaced modifier?

a. the trail up the mountain winds alongside a rushing stream.
b. during the climb, mr. swanson slipped on a loose rock up a steep hill.
c. by late afternoon, they arrived in a meadow and set up camp.
d. the swansons sat on a rock beside the stream and had a snack.

(i really need fast . i don't understand this)

the president was the speaker and it was a great part of history.

explanation:

video of the national aeronautic space administration's (nasa) coverage of president john f. kennedy’s address at rice university, houston, texas, concerning the nation's efforts in space exploration. in his speech the president discusses the necessity for the united states to become an international leader in space exploration and famously states, "we choose to go to the moon in this decade and do the other things, not because they are easy, but because they are hard."