How to turn acetaldehyde into acetic acid using one of the five chemicals on the list? From the Russian State Chemistry Test:


  


It's easy to see $X$ is $\ce{H2O}$: water turns acetylene into acetaldehyde. 

But what can turn acetaldehyde into acetic acid? 

I checked: $\ce{NaOH}$ would turn it into sodium acetate, $\ce{CuO}$ would not react (you'd need $\ce{Cu(OH)2}$), ethanol would turn it into either an acetal or hemiacetal.

Would acetaldehyde react with formic acid to produce acetic acid? I tried to google for it, and found no mention. 

Would water in the presence of a heat source somehow react with acetaldehyde? I googled and discovered that it might form hydrates, not acetic acid.

So I'm at a loss. If you consider the question too easy, you could give just a hint.
Oxidation of aromatic aldehydes to acids by air oxidation catalysed by CuO is reported in Molecules 2008 13 948

None of the other reagents can oxidise aldehyde to acid.