When Br2 is added to buta-1, 3-diene at - 15 C, the product mixture contains 60% of

Question:

When Br2 is added to buta-1, 3-diene at - 15 °C, the product mixture contains 60% of product A and 40% of product B. When the same reaction takes place at 60 °C, the product ratio is 10% A and 90% B.
(a) Propose structures for products A and B.
(b) Propose a mechanism to account for formation of both A and B.
(c) Show why A predominates at - 15 °C, but B predominates at 60 °C.