易患免疫系统疾病如急性肾炎、风湿热等的年龄阶段是（）
A.新生儿期
B.婴儿期
C.幼儿期
D.学龄前期
E.学龄期
参考答案：D
如图，球A、B置于光滑水平面上，A球的动量为12kg•m/s，水平向右与静止的B球发生碰撞，两球动量的变化可能是（设向右为正）（ ）
A．△pA=-4kg•m/s，△pB=4kg•m/s
B．△pA=-5kg•m/s，△pB=5kg•m/s
C．△pA=6kg•m/s，△pB=-6kg•m/s
D．△pA=-24kg•m/s，△pB=24kg•m/s
—Kim, I haven’t seen your mother for so long, where is she?
—Oh, she ________________ England to learn further studies(深造). She left a month ago.
A．have been to
B．has gone to
C．has been in
D．have been
