much study have not be done vs much study has not been done

Some examples from the web:

• How exactly this should be done is a matter which still requires much study and consultation.
• Secondly, in practice, too, this can be done without too much difficulty, as is demonstrated by a study to which even the German Minister for Health has referred.
• Studies have shown that cramming can improve short-term performance.
• Specific drug-drug interaction studies have not been conducted with zoledronic acid.
• No specific drug-drug interaction studies have been conducted with zoledronic acid.
• Animal studies have shown excretion of lasofoxifene in milk.
• Animal studies have shown excretion of anidulafungin in breast milk.
• No in vivo human pharmacokinetic interaction studies have been performed.
• - No pharmacokinetic interaction studies have been completed.
• Animal studies have not revealed teratogenic effects.
• Specific in vivo drug interaction studies have not been performed.
• Interaction studies have only been performed in adults.
• No interaction studies have been performed in humans.
• Animal reproduction studies have not been conducted.
• Studies have shown that timolol does not dialyse readily.
• Animal studies have shown excretion of brinzolamide in breast milk.
• No formal interaction studies have been performed with Beromun.
• Long term carcinogenicity studies have not been performed.
• Long-term animal carcinogenicity studies have not been performed.
• Animal studies have shown excretion of vildagliptin in milk.

Some examples from the web:

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