吸烟和心脏病之间的联系是众所周知的。

The ___connection between smoking and heart disease is well known.

他否认与恐怖分子有任何关联。

He has denied having any ___connection with the terrorists.

她们是姐妹，是吗？我知道她们的姓氏相同，但我从未联系起来。

They're sisters, are they? I knew their surname was the same, but I never made the ___connection.