单选题 The girl was heard ______ upstairs.
[A] to sing [B] sing [C] singing

【正确答案】 C
【答案解析】[解析] 本题考查宾语补足语。hear sb. doing sth. 意为“听见某人正在做…”。本句是被动语态，sb. be heard doing sth. 。