1. You can determine the names of districts and counties in Shanghai. Any place name can tell whether it is the name of a district or county in Shanghai. So this is a collection.
2. Natural numbers with a mantissa of 3 are also determined by concepts. Any number can know whether he is a member of this group according to this rule. So it is a collection.
3. For students whose height is greater than 1.70, this concept is also certain. A person is either above 1.70 or above 1.70. So we can clearly know whether a classmate is a member of this group, so it is a collection.
The three can form a set.
Non-zero natural numbers are positive integers.