5月31日是“世界无烟日”，香烟的烟气中含有几百种有毒物质，其中有一种极易与血液中的血红蛋白结合，这种物质是 (      )

A．CO₂

B．SO₂

C．CO

D．NO₂

The most interesting thing to the small boy was _______ he got close to the door, it would open of itself.

A．that as

B．that that

C．what that

D．what when