下列有关免疫的叙述，正确的是（　　）

东汉后期和唐朝前期黄河流域、长江流域县城数量表（单位：座）

概述上表反映的县城数量及分布的变化，阐释导致变化的历史原因。

    It was Sunday today. Tom with her two good friends are going to see his uncle . There are lots of apples trees on his uncle farm. But he is too busy to pick the apples in them. So Jim went to help him every year. The farm is no far from here. The children are all going there on bike . They think it's good for their healthy.

伞式洋流发电是通过“伞”带动工作索周而复始的运动，进而带动发电机运转发电。读“美国本土南部附近海域的伞式洋流发电站示意图”。完成下列问题。

洋流G的性质和地理位置可能是（   ）
①暖流    ②寒流    ③大洋东岸    ④大洋西岸