1. 下面哪些语句会使用到索引，哪些索引起了作用？
```
CREATE TABLE t1 (
id int unsigned NOT NULL auto_increment,
a int unsigned NOT NULL DEFAULT 0,
b int unsigned NOT NULL DEFAULT 0,
c int unsigned NOT NULL DEFAULT 0,
PRIMARY KEY(id),
KEY abc (a, b, c) USING BTREE
);
(1). SELECT * FROM t1 WHERE a=3 AND b=5 AND c=4;
(2). SELECT * FROM t1 WHERE c=4 AND b=6 AND a=3;
(3). SELECT * FROM t1 WHERE a=3 AND c=7;
(4). SELECT * FROM t1 WHERE a=3 AND b>7 AND c=3;
(5). SELECT * FROM t1 WHERE b=3 AND c=4;
(6). SELECT * FROM t1 WHERE a>4 AND b=7 AND c=9;
(7). SELECT * FROM t1 WHERE a=3 ORDER BY b;
(8). SELECT * FROM t1 WHERE a=3 ORDER BY c;
(9). SELECT * FROM t1 WHERE b=3 ORDER BY a;
```
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