The message is very important，so it is supposed as soon as possibleA. to be sentB. to se

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A. to be sent
B. to send
C. being sent
D. sending
解析：此处be supposed to do sth．意为“被期望做某事；应该做某事”，不定式做宾语。表示“消息被传送”，应该用不定式的被动式。下列动词常跟不定式做宾语：want，wish，hope，expect，ask，demand，pretend，care，decide，happen，offer，refuse，fail，plan，prepare，order，cause，afford，beg，manage，agree，promise等。