Read the following passage and mark the letter A, B, C, D on your answer sheet to indicate the correct answer to each of the questions.

Face-to-face conversation is a two-way process: You speak to me, I reply to you and so on. Two-way (18)_______ depends on having a coding system that is understood by both sender and (19)_______, and an agreed convention about signaling the beginning and end of the message. In speech, the coding system is the language like English or Spanish; the convention that one person speaks at a time may seem too obvious to mention. In fact, the signals (20)_______ in conversation and meetings are often (21)_______. For example, lowering the pitch of the voice may mean the end of a sentence, a sharp intake of breath may signal the desire to interrupt, catching the chairman’s eye may indicate the desire to speak in a formal setting like a debate, a clenched fist may indicate anger. When (22)_______ visual signals are not possible, more formal signals may be needed.

Điền ô số 22

A. their

B. These

C. This

D. That