Leaving out 'which is' any time is dangerous, because it is more often used for non-restrictive clauses than restrictive clauses, and non-restrictive clauses to not permit zero- that/which In restrictive clauses, the relative pronouns that and which (+ be ) can be omitted if no confusion in meaning is caused and if: 1-- The that -clause is a direct object or complement: I see he's coming; I know I'm early . 2-- The that-clause is a right-shifted subject with an anticipatory it: It's too bad he's not here . Otherwise, you should retain that/which .
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