题目

 下列结构中，杨树的细胞具有而大雁的细胞不具有的是                A．细胞壁               B．细胞膜            C．细胞质            D．细胞核 答案：A_____ the project in time, the staff were working at weekends. [     ]A. Competing B. Having completed C. To have completed D. To complete