Let a= { 15, 25, 35, 45, 55, 65 } and b= { 25, 45, 65} what is a∪b?

choices:
a) {15, 35, 55}
b) { 25, 45, 65}
c) { 15, 25, 35, 45 }
d) {15, 25, 35, 45, 55, 65 }

Sound travels about 750 miles per hour. a) suppose it takes about 1.5 seconds to hear the echo. how far away is the canyon wall, in feet? for this i got 825 ft b) suppose you stand 1,925 feet from the canyon wall and sound the horn. how long will it take to hear the echo? (i'm stuck on this)