Read the following passage and mark the letter A, B, C or D to indicate the correct word or phrase that best fits each of the numbered blanks.

The most dominant and reliable features of facial expressions (17) a constant channel of communication. They can be shifty and evasive; convey hate, fear, and guilt; or  express confidence, love, and support. Referred to as "mirrors of the soul" our eyes serve as the major decisive factor in interpreting  the spoken words. The eyes of the man converse as much  as their tongues, with the advantage (18) the ocular dialect needs no  dictionary, but it is understood all over the world. When the eyes say one thing, and the tongue another, a practiced man relies on eyes. Except (19) extremely shy individuals, most people look for social acceptance by studying the eyes of others. Eyes also can accurately indicate a positive or a negative (20). People tend to look longer and more often at the people whom they trust, respect and care about than at those whom they doubt or  dislike.

The most dominant and reliable features of facial expressions (17) a constant channel of communication.