Making bubbles

This task is about fair testing.
dishwashing liquid.png
Ana and Jen want to find out which is the best washing-up liquid for blowing bubbles.
Help them to plan a "fair test".

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a)  What does "best" mean in the circumstances?
     

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b)  How many types of washing-up liquids should they try?
     

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c)  What should they measure?
     

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d)  What will they need to keep the same?
     

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e)  What should they do to make sure their results are more accurate?