Costa 's league - leading 12th goal of the season extended Chelsea 's Premier League winning streak to nine .
Costa 's league - leading 12th goal of the season returned them to the top of the table with victory over West Bromwich Albion .
Costa 's league - leading 12th goal of the season extended Chelsea 's Premier League winning streak to nine and returned them to the top of the table with victory over West Bromwich Albion .