根据下面资料，回答47-50题
bid your child's brain become smaller last summer? Probably not, according to a study byDr. Harris Cooper, professor at the University of Missouri-Columbia. The study found that when studentsreturn to school after a long summer vacation, they' ve lost one to three months worth of learning.
This is more harmful for math than it is for reading. "All students lose math skills," says Cooper.It may be because kids (小孩) have more chances to practice reading than math. Middle-class childrenactually learn more in reading than lower-income students over the summer. Cooper feels that this resultsfrom the variety of activities that many middle-class kids take part in over the summer, such as camping (野营) and trips.
"Parents can help their kids keep educational skills," says Cooper. He suggests the following tipsto have a learning-filled summer.
1. Keep lots of books around and make regular trips to the library.  Most libraries offer special summer events for kids.
2. Think about what your kids may be learning next year when you plan the family vacation. Talk with teachers to find out what they'll be covering in class.
3. Keep math in mind. Since kids lose more math skills than anything else over the summer, try to do some special planning to find math-related activities.
4. Consider summer school. Struggling kids can get a lot of different kinds of help from summer school programs. Summer school can also enrich and quicken learning in areas where kids show a special interest.
Remember to keep it fun! You don' t want to make your kids unhappy with learning during the summer break.
Some kids gain in reading over the summer probably because they_______.

A  join in a variety of activities

B go to the library every day

C  spend their vacation in a school

D receive a special training in reading