V. Choose the word (A, B, C or D) that best fits the blank space in the following passage.

Television first (17) _______________ some fifty years ago in the 1950s. Since then, it has been one of the most (18) _______________ sources of entertainment for both the old and the young. Television offers cartoons for children, world (19) ______________, music and many other programs. If someone is interested in sports, for instance, he can just choose the right sports channel. There he can enjoy a broadcast of an international football match (20) _______________ it is actually happening. Television is also a very useful way for companies to advertise their products.
It is not too hard for us to see why (21) _______________ is a TV set in almost every home today. And, engineers are developing interactive TV (22) _______________ allows communication between viewers and producers.