The mother felt herself       cold and her hands trembled as she read the letter from the         battlefield.

A．grown

B．grow

C．to grow

D．to have grown

出售某种文具盒，若每个获利x元，一天可售出（6﹣x）个，则当x=（    ）元时，一天出售该种文具盒的总利润y最大．