Either they can't answer in public or they will spill the beans

No pleading the fifth as they aren't under oath.
Collusion
Obstruction of Justice
Live Testiminy today from the Director of National Intelligence Daniel Coats and National Security Agency Director Michael Rogers who Trump told to publicly deny that there is any evidence of connections between Trump?s team and Russia.
Those requests reportedly came in late March, after then-FBI Director James Comey confirmed that his bureau was looking into the Trump campaign?s ties to Russia.
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Trump Reportedly Asked Intel Chiefs To Publicly Deny Russia Ties | HuffPost
Amid all this, Trump and the White House have denied any wrongdoing and have argued the investigation is detrimental to the country.
?I believe it hurts our country terribly because it shows we?re a divided, mixed-up, not-unified country,? Trump said Thursday. ?And we have very important things to be doing right now, whether it?s trade deals, whether it?s military, whether it?s stopping nuclear ― all of the things that we discussed today. And I think this shows a very divided country.?