二叉树由三个基本单元组成，即左子树，右子树和 【4】 。
参考答案：根节点
解析： 二叉树由根节点、左子树和右子树组成。
I was told that his uncle had gone to France . [ ]
A. long before
B. shortly after
C. before long
D. long ago
在沉积地层中（）油层，注入水一般在油层内推进状况比较均匀。
A.正韵律
B.反旋回
C.正旋回
D.符合旋回
