My mother is good at _____ . She is a good dancer.
dancing
to dance
dance
dances
be good at +doing
be good at 意为“擅长.....”，其中，at是介词，其后跟动名词形式作宾语，故选A。
— _____ the twins in the library now?
— No,they_____.
Do; don't
Are; aren't
Is; isn't
Are; don't
be动词
由句子结构可知，此处应用be动词，问句的主语是the twins为复数形式，故be动词用are。由问句可知答语中用aren't，所以答案为B。