Choose the word (a, b, c, or d) that best fits each of the blank space

Choose the word (a, b, c, or d) that best fits each of the blank spaces. Kenji and Ykari are new students in Peter's class. They are from Japan. They said they (Question 21) ___ together, (Question 22) ___ a week. They studied their textbook (Question 23) ___ it has the grammar at the back. They learned the rules (Question 24) ___ heart and then they wrote (Question 25) ____ of sentences and examples, and practiced with conversations. Question 21.