The key answer of exercise 5

a) Four

1. in fridge → to remind you when it's time to buy something
2. babies → to monitor a person all their lives
3. criminals → to keep track of them
4. attached to any object → to help you find it when you lose it

b)

1. they can be placed (this keeps microchips as the main focus)
2. you can be reminded (now the focus shifts to 'you' and what 'you' do, i.e. it's not focusing primarily on the microchips anymore)
3. you to buy something (keep the focus on 'you')
4. microchips could be implanted (keep the focus on microchips, the main topic of the whole text)
5. Microchips could also be implanted (keep the focus on microchips)
6. keep ('police is the subject or agent of the verb, so it needs an active verb)
7. a crime is committed (we don't know who commits the crime)
8. you can buy a set of clip-on microchips (the new focus is on 'you' established at the beginning of this sentence)
9. both answers are possible, but 'can be attached' sounds better since the focus has just shifted to 'a set of clip-on microchips'
10. both answers are possible; the first ('you can use your phone') sounds slightly less formal