1.This exam is a real c_____.Please study hard for it.2.Most

1.This exam is a real c_____.Please study hard for it.2.Most children can not c___ on one thing fo
1.This exam is a real c_____.Please study hard for it.
2.Most children can not c___ on one thing for a long time.
3.Don't you remember ____(borrow) 100Yuan from me?