Class is over. The students stop and start .
A:writing; to talk
B:to write; talk
C:writing; talk
D:to write; talking
下列物质中属于混合物的是 ( )
科学家依据基因工程的原理最新研制的某种疫苗是用病原微生物编码抗原的基因制成的,这个基因编码的产物能诱导机体产生免疫反应。该疫苗
已知y=2x-5+5-2x-3,则2xy的值为( )