题目

---What _______ you ________ ?        ---I’m doing my homework. A. do; do               B. are; do             C. are; doing     D. can; do 答案：C已知2a=3，2b=6，2c=12，则下列关系正确的是（　　）A．2a=b+cB．2b=a+cC．2c=a+bD．c=2a+b