In the following sentence, one numbered part may contain a grammatical error, or the sentence may be fully correct. Choose the number of the part that has an error, or choose option (4) for No Error. Sentence: Radium is so expensive that you (1) / could be a millionaire if you own (2) / only half a pound of it. (3)

Introduction / Context:
This sentence tests conditional structures in English, especially the matching of verb forms between the if clause and the main clause. The statement presents a hypothetical situation about the high price of radium and how owning a small quantity could make a person very rich. The learner must detect the mismatch between the modal verb could and the present tense own in the conditional clause.

Given Data / Assumptions:

• The sentence is divided into three numbered parts, followed by a No Error option.
• Part (1) states the condition that radium is very expensive and begins the result clause with you could be a millionaire.
• Part (2) is the if clause with if you own.
• Part (3) completes the idea with only half a pound of it.

Concept / Approach:
The key concept is type two conditional, which describes unreal or hypothetical present situations. In such sentences, the if clause normally uses a past tense form (for example if you owned) while the main clause uses would or could plus the base verb (for example you could be). Here the main clause uses could be, which signals a hypothetical meaning, but the if clause incorrectly uses the present tense own instead of owned. Therefore, the correct structure should be could be a millionaire if you owned only half a pound of it. The error lies in part (2).

Step-by-Step Solution:
Step 1: Read part (1): Radium is so expensive that you could be a millionaire. This part sets up a hypothetical result and is acceptable as written.Step 2: Read part (2): if you own. This if clause uses the present tense own, which usually describes a real or general situation.Step 3: Recognise that the sentence is not talking about a real present fact but about a hypothetical situation of owning radium.Step 4: Recall that type two conditionals use a past tense in the if clause, so the correct form would be if you owned.Step 5: Read part (3): only half a pound of it. This completes the idea and does not contain any error by itself.Step 6: Conclude that part (2) contains the tense error and should be changed to if you owned.

Verification / Alternative check:
To verify, rewrite the full sentence with the corrected tense: Radium is so expensive that you could be a millionaire if you owned only half a pound of it. This version fits the pattern of hypothetical conditional sentences: if plus past tense in the condition, and could plus base verb in the result. Another acceptable pattern would be you would be a millionaire if you owned only half a pound of it, which again shows that owned is the correct form in the if clause. Since no changes are needed in the other parts, the error is clearly located in segment (2).

Why Other Options Are Wrong:
Option 1: Part (1) uses Radium is so expensive that you could be a millionaire, which is a natural way to introduce a hypothetical result and does not contain a grammatical error.Option 3: Part (3) only provides the specific quantity only half a pound of it and is structurally correct.Option 4: Because the conditional clause in part (2) is incorrect, the entire sentence cannot be considered error free, so No Error is not the correct choice.

Common Pitfalls:
Learners often focus on the price related vocabulary and miss the subtle tense mismatch. Another common mistake is to assume that any clause beginning with if can use present tense by default, because in many real conditionals we do say if you own, if it rains, and so on. However, when the main clause uses would or could to indicate an unreal situation, the if clause should switch to a past tense. Recognising the link between modal verbs in the main clause and tense choice in the if clause is essential for these questions.

Final Answer:
The incorrect tense appears in the second part, so the correct option is 2.