The key answer of exercise 11

a)

1. dropped alarmingly
2. a drop
3. plummeted
4. surge; declined
5. collapsed
6. sharp

b)

1. There has been an explosion in (the) demand for mobile technology in language learning.
2. The number of people communicating regularly using social networks has increased dramatically.
3. The number of students attending private language schools to study English has plummeted.
4. There has been a sharp increase in the ability of learners to access learning materials on the internet.
5. There has been a gradual decline in the appeal of traditional teaching methods.