Which of the following lines for call of the wild best supports one of the themes?
a."for the pride of trace and trail as his, and sick unto death, he could not bear that another dog should do his work."
b. "he knew he was at last answering the call, running by the side of his wood brother toward the place from where the call surely came."
c. "as twilight fell the old bull stood with lowered head, watching his mates-the cows he had known, the calves he had fathered, the bulls he had mastered-as they shambled on at rapid pace through the fading light."
d. "here a yellow stream flows from rotted moose-hide snacks and sinks into the ground, with long grasses growing through it and vegetable mound overrunning it and hiding its yellow from the sun; and here he muses for a time, howling once, long and mournfully, ere he departs."