A：-23
B：23
C：-3
D：3
—Jenny and Joe have________recently.
—How come？As far as I know，they have been good friends for many years.
下列各种动物行为中，不属于学习行为的是
quantities of, on average, on behalf of, in existence, on the whole, in panic, in appreciation of, be apposed of, decide on, in spite of
