题文
Scientists ______ thousands of chemicals before they decided on the right one in the end.
| A．tried out |
| B．found out |
| C．worked out |
| D．turned out |
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Scientists ______ thousands of chemicals before they decided on the right one in the end.
| A．tried out |
| B．found out |
| C．worked out |
| D．turned out |
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