This weekend is Tim's ____(nine) birthday, he asked his father to buy him ____new ipad. But this worries Tim's father much, can Tim use the ipad ____ (correct)? Will it be a good or bad present for the little boy?

    We all know that the computer is a great ____ (invent) in the world, it is very ____ (use) for people's life. It's interesting for people ____ watch movies or play games with ipads or computers. Now ____ (million) of people are using it every day.____it also may be bad to young people in some ways. Some children are very ____ (interest) in playing games or reading books, it may hurt ____ (they) eyes and make their study worse. All the parents hope their children can find better ways of using it.

假如某大肠杆菌含¹⁴N的DNA的相对分子质量为a，若将其长期培养在含¹⁵N的培养基中，便得到含¹⁵N的DNA，相对分子质量为b。现将含¹⁵N的大肠杆菌再培养在含¹⁴N的培养基中，那么，子二代DNA的相对分子质量平均为（  ）