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.

Face-to-face conversation is a two-way process: You speak to me, I reply to you and so on. Two-way (26)_______ depends on having a coding system that is understood by both sender and (27)_______, 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 (28)_______ in conversation and meetings are often (29)_______. 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 (30)_______ visual signals are not possible, more formal signals may be needed.