在PostgreSQL中，请看下面的空表：
CREATE TABLE example (
a integer,
b integer,
c integer,
UNIQUE (a, c)
);
以下哪项插入会导致错误？( )
insert into example (a, b, c) values (1, 2, 3), (1, 2, 4);
insert into example (a, b, c) values (1, 2, 3), (3, 3, 3);
insert into example (a, b, c) values (1, 1, 1), (3, 3, 3);
insert into example (a, b, c) values (1, 2, 3), (1, 4, 3);
都不会导致错误
