Why was Snape teaching DADA?

[anujaasathe]

No: HPFGUIDX 135899

I am not sure why we saw Snape teachind DADA this year.Is it: 
1.because Dumbledore finally tusts him so much that he thinks it wont 
do any harm
2.because he cant (yet again)find a DADA teacher but knows he can
find 
another one for potions.so after getting a new teacher for potions he 
had no choice but to let Snape teach DADA?
3.or because he always intended to get the memory from Slughorn and
so 
needed him at Hogwarts?

Anuja.