All of the MBTA Green Line's downtown portion and much of its branches will be closed for 15 days in December to fix a nearly 130-year-old tunnel piece, the transit agency announced. From Dec. 8 through 22, the Green Line's main branch will be closed from Kenmore to North Station. The E Branch will be entirely closed, and the B Branch will run from Boston College to Babcock Street, with shuttle buses running from Babcock to Back Bay. The C and D Branches will run only between their terminuses and Kenmore.