Let's analyze each blank and choose the most appropriate word or phrase:

**Blank 19**:
The context here is about scientists creating new gadgets and equipment.
- A. invent (correct: "invent" means to create something new)
- B. discover (not quite right; "discover" often refers to finding something that already exists)
- C. think (not suitable in this context)
- D. look (doesn't fit the context)

**Blank 20**:
The sentence refers to homes being full of technological devices.
- A. off (not correct in this context)
- B. of (correct: "full of" is the correct phrase)
- C. with (not the right preposition here)
- D. for (doesn't fit the context)

**Blank 21**:
The sentence compares the ease with which teenagers use technology to that of older generations.
- A. their (this would be correct if it matched the subject "mums and dads and grandparents")
- B. his (incorrect as it is singular and does not match the plural subjects)
- C. her (incorrect as it is singular and does not match the plural subjects)
- D. its (incorrect as it is singular and does not match the plural subjects)

**Blank 22**:
The sentence talks about one's ability to handle new technology in the future.
- A. quantity (incorrect; "quantity" refers to the amount of something)
- B. quality (incorrect; "quality" refers to the standard of something)
- C. facility (incorrect; "facility" refers to a place or the ease of doing something)
- D. ability (correct: "ability" refers to the capability to do something)

Therefore, the correct answers are:
19. A. invent
20. B. of
21. A. their
22. D. ability

So the passage with the blanks filled in would be:

Modern technology is changing and improving all the time. Every month, scientists **(19) invent** new gadgets and equipment to help us with our daily lives and discover ways to make existing technology faster and better. Our homes are full **(20) of** hardware and software. Some researches suggest, however, that it's young people who are best able to deal with this change. While teenagers have no problem operating a DVD player, **(21) their** mums and dads and grandparents often find using new technology complicated and difficult. If you're a teenager who criticizes your parents for their lack of technological awareness, don't be too hard on them! Sometimes in the future, when you've got children, your **(22) ability** to deal with new technology will probably decrease and your children will be more comfortable with new technology than you are. You won't want them to criticize you, will you?