Where would you see the abbreviation 'ps" а) at the foot of the letter б) on a compass в) on a toilet door

At the foot of the letter (в конце письма).

Real book: advantages 1. you can feel texture of paper. 2. it not required a charge. 3. you can smeel it. 4. when you read a real book, you are experiencing a brighter story. 6. we can mentally travel through pages 7.  reading information from paper pages is easier for the eyes. disadvantages1. you can crumple pages2. easily torn cover3. requires a light source4. text can fade5. it is not always easy to find in a store or library6. it's easy to forget which page stopped7. you need storage space e-book: advantages1. e-books  weightless2. e-books  are convenient, you can increase the font or brightness3. e-books save your budget4. e-books do not require wood that is used for pages5. you can store many books6. you can customize it for yourself7. reading continues from the place where you left off disadvantages 1. it must be periodically recharged2. not everyone can read from the screen3. cost more than a paper book4. limited storage capacity5. it may break6. you can lose it 7. not all books are in electronic form