safe vs unhealthy

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Senior Member
Hi everybody. :)

When dealing with exercise, I encountered one question. :) Now could I ask you this, please? :)

These are four sentences in the passage about 'mobile phone'.

Mobile phones emit microwave radio emissions. Researchers are questioning whether exposure to these radio waves might lead to brain cancer.
So far, the data are not conclusive. The scientific evidence does not enable us to say with certainty that mobile phones are categorically (1) ______.

1. A. risky
B. secure
C. safe
D. unhealthy

The key is C but could you tell me why not D, please? :) Please help me out. Any help would be greatly appreciated. :)
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