Optional selection for Brass brush, nature hair brush.

有棕刷、毛刷任选一种。

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Hence, it is very important for optional selection the additive preventing the gas flow effectively, and developing good cement slurry for preventing the gas flow.

因此，优选有效的防气窜添加剂，研制性能良好的防窜水泥浆就显得十分重要。

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Additionally, determine how making a specific optional field into a required field or adding a new value to a selection list can be accomplished.

此外，还要确定如何把某个可选字段变为必需字段或者向选择列表增加的新值。

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Academic estimates are that these statistical biases of self-selection (reporting is optional for the largely unregulated hedge fund industry) add 3-5 percentage points to index returns.

FORBES: Chasing the Mirage of Hedge Fund Returns