old前面加a还是an

　　例句：

　　The city is dissected by a network of old canals.

　　古老的运河网将这座城市分割开来。

　　She comes from an old Scots family.

　　她出生于一个古老的苏格兰家族。

　　My new job is much the same as the old one.

　　我的新工作和旧工作差不多。

　　I'd say he was 25 to 30 years old.

　　我猜他在25至30岁之间。

　　Now let me see ─ how old is she now?

　　让我想一想，她现在多大了呢？