Has company B breached the contract? Why?

7. A and B company signed a contract for the sale of house, company A shall deliver 100 sets of houses and go through the formalities of registration in 1 September, company B shall pay to company A in three payments. The last 50 million yuan shall be paid when company A delivered the houses to company B in September 1st. After signing the contract, the company B paid a total of 50 million yuan for the first and second payment on schedule. In September 1st, company A transferred the keys of the houses to the company B, but did not immediately register for the transfer of property ownership. Therefore, the company B says the remaining amount will be paid after the registration is completed. 7 days after the date of the contract payment (September 1st), company B still has no payment, company A then sued company B to the court, and requested company B to bear the liability for breach of contract. Has company B breached the contract? Why? (20')