B4

Have you got a mobile phone?

Before mobile phones were developed, you could only call someone with a phone which was connected to a phone line.

(past simple passive, т.к. говорится об определённом периоде в прошлом)

DEVELOP

B5

That problem meant that it was often difficult to contact people, particularly if you were on a bus or in a car.

(past simple, т.к. говорится об определённом периоде в прошлом)

MEAN

B6

Before mobile phones, people didn’t send text messages to each other.      

(past simple, т.к. говорится об определённом периоде в прошлом)

NOT SEND

B7  

Since they were introduced, text messages have changed the way we communicate.

(present perfect, т.к. они изменились и продолжают меняться)

CHANGE

B8

With text messages, you don’t need to actually call someone.

(present simple, т.к. общее утверждение)                   

NOT NEED

B9

You simply type a short message and send it, being careful, of course, to send it to another mobile and not to a home phone.

(Participle I, “будучи осторожной”, подходит по смысловому контексту)

BE

B10 

Mobile phones used to be big and heavy. Now, they're small and light.

(used to - когда-то было)

USE

B11

The chances are that they will become even smaller and lighter in the future.

(станут)

BECOME