How is the taiga important to birds from other biomes? why can’t the summer birds live in the taiga year round?

The taiga is very important for the summer birds as a biome because it provides them a more pleasant climate and sufficient amount of food during the summer period when in the biomes on the south it is much hotter and the food and water are much less available. But the taiga only provides these suitable conditions during the summer, while in the biggest part of the year it is extremely cold, with temperatures going down to -40 and -50 degrees regularly, the ground is frozen as are the tree branches, and the food is extremely hard to be found and is very limited, so the birds have to return to the southern parts as the end of the summer in the taiga comes.

the answers are a and c