Question 77 - 79 refer to following paragraph:

W: Attention, passengers. Our arrival in Chicago is expected to be somewhat behind schedule. (77) Due to the heavy snowfall, our bus driver must use appropriate caution and drive at a slower speed. (78) Therefore, we will probably be arriving an hour later than our scheduled arrival time, which was 5:00 P.M. Although these circumstances are out of our control, we do apologize for any inconvenience it may cause you. (79) We would like to remind passengers that this bus offers a free Wi-Fi connection. This is just one of the amenities that make riding with us more comfortable than with our competitors.

77. What has caused the problem?