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When adding soluble oil to the machine, it is crucial to mix it with water before adding it to the coolant system. This method ensures that the soluble oil properly emulsifies with the water, creating a suitable coolant mixture that offers effective lubrication and cooling during machining processes. If soluble oil is poured directly into the coolant tank without dilution, it may not mix properly with the existing coolant or may lead to an ineffective combination, adversely affecting the machining performance and potentially damaging the equipment. Additionally, using soluble oil only in extreme cases contradicts the purpose of having it as a standard coolant option, as it is designed to enhance lubricity and cooling under normal operating conditions. Thus, the most appropriate practice is to prepare the mixture before addition, leading to the correct understanding that "none of these answers is correct" is indeed not the right choice in this context.