In 1971 T.O. Diener discovered a new infectious agent that was smaller than viruses-
I. It causes potato spindle tuber disease
II. It is free RNA
III. Molecular wt. of RNA is low
The above statements are assigned to-

Options

A. Viruses
B. Viroids
C. Virulent
D. Mycoplasmal

Answer

To solve the question regarding the discovery by T.O. Diener in 1971, we need to analyze the statements provided and identify the infectious agent being referred to.

### Step-by-Step Solution:

1. **Identify the Discovery**: T.O. Diener discovered an infectious agent in 1971 that was smaller than viruses. This agent is known as a viroid.

2. **Evaluate Statement I**: The first statement claims that this agent causes potato spindle tuber disease. This is true; viroids are known to cause diseases in plants, including potato spindle tuber disease.

3. **Evaluate Statement II**: The second statement mentions that it is free RNA. This is also correct; viroids consist solely of a short strand of circular RNA and do not have a protein coat, distinguishing them from viruses.

4. **Evaluate Statement III**: The third statement states that the molecular weight of the RNA is low. This is accurate; viroids have a low molecular weight compared to other nucleic acids.

5. **Conclusion**: All three statements (I, II, and III) accurately describe the characteristics of viroids. Therefore, the statements are assigned to viroids.

### Final Answer:
The above statements are assigned to viroids.

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