The correct options are
A The sap from the trees stops flowing making it easier to cut.
B The felled logs could be easily collected on the frozen grounds and taken to the river banks.
C It is difficult to cultivate crops in winters and so lumbering is done as an alternative activity.
Lumbering activity is carried out in winter when the ground is covered in snow so that the cut logs can easily be hauled over the frozen ground and piled on the frozen rivers to be transported later when the ice melts in spring. Also, in winters, the sap from trees stops flowing, making it easier to cut, and it isn't very easy to cultivate crops in winters. So lumbering is done as an alternative activity.