1. Today, millions of people have one form of religion, and their neighbors a different one.

2. Today, millions of people have one form of religion, and their neighbors a different religion.

3. Today, millions of people have one form of religion, and their neighbors a different one of religion.

4. Today, millions of people have one form of religion, and their neighbors have a different one of religion.

5. Today, millions of people have one form of religion, and their neighbors have a different form of religion.
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Are they all grammatical? What is the full form of #1? Does 'one' at the end in Sentence 1 refer to 'religion'?

1, 2, and 5 are grammatical.

The full form of # 1 is # 5. Yes, "one" refers to religion.

1. "Today, millions of people have one form of religion, and their neighbors a different one."

This sentence is grammatically correct and the full form of #1 would be: "Today, millions of people have one form of religion, and their neighbors have a different form of religion."
In this sentence, the word "one" at the end refers to "form," not "religion." It means that the people have one particular form of religion, while their neighbors have a different form.

2. "Today, millions of people have one form of religion, and their neighbors a different religion."
This sentence is grammatically incorrect. It should be revised to: "Today, millions of people have one form of religion, and their neighbors have a different religion."

3. "Today, millions of people have one form of religion, and their neighbors a different one of religion."
This sentence is grammatically incorrect. It should be revised to: "Today, millions of people have one form of religion, and their neighbors have a different form of religion."

4. "Today, millions of people have one form of religion, and their neighbors have a different one of religion."
This sentence is grammatically incorrect. It should be revised to: "Today, millions of people have one form of religion, and their neighbors have a different form of religion."

5. "Today, millions of people have one form of religion, and their neighbors have a different form of religion."
This sentence is grammatically correct.