A cause-and-effect relationship is signaled by the words "But since."
I believe he is drawing a parallel between a tone of pure joy and the most depressing despair.
1 (D) The loneliness of human beings
2 (A) The heron feeding on the snake in the pond
3 (D) The rabbits are part of George and Lennie's dream of one day having their own farm
4 (A) Flashback
5 (A) Time order/sequence signal words
6 (C) As a point of tension in the plot's rising action
