
- A$80^\circ$
- ✓$90^\circ$
- C$70^\circ$
- D$100^\circ$


In the figure, $\text{AB}\parallel\text{CD}$
$\angle\text{OAB}=150^\circ,\angle\text{OCD}=120^\circ$
From $O$, draw $\text{OE}\parallel\text{AB}$ or $CD$
$\text{AB}\parallel\text{DE}$
$\angle\text{OAB}+\angle\text{AOE}=180^\circ$
$\Rightarrow150^\circ+\angle\text{AOE}=180^\circ$
$\Rightarrow\angle\text{AOE}=180^\circ-150^\circ=30^\circ$
Similarly $\text{DE}\parallel\text{CD}$
$\Rightarrow\angle\text{EOC}+\angle\text{OCD}=180^\circ$
$\Rightarrow\angle\text{EOC}+120^\circ=180^\circ$
$\Rightarrow\angle\text{EOC}=180^\circ-120^\circ=60^\circ$
Now $\angle\text{AOC}=\angle\text{AOE}+\angle\text{EOC}$
$=30^\circ+60^\circ=90^\circ$
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