When I stayed in the countryside I enjoyed _____ leisurely walk in the fields, and felt _____ warmth

of the sun on my face.[      ]

 A. /; the          

B. a; a      

C. /; a          

D. a; the

下列物质由离子构成的是( )

A．汞

B．氮气

C．氯化钠

D．二氧化碳