I presume that you are confronted with this problem, which is a common one for learners: Use 'which' for nonrestrictive (non-defining) clauses that offer extra information about the referent: I like my bicycle, which has a really cool seat . Use 'that' or 'which' for restrictive (defining) clauses which are necessary to define the referent: I like the bicycle that/which I just bought . Many grammarians prefer 'that' in the second case, but the guideline is clarity.
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