With the word “PM2. 5” constantly __________ in media reports, you might be wondering what on earth it means.
| A．appeared |
| B．appearing |
| C．to appear |
| D．being appeared |
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With the word “PM2. 5” constantly __________ in media reports, you might be wondering what on earth it means.
| A．appeared |
| B．appearing |
| C．to appear |
| D．being appeared |
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