B: No, they .......... They .......... at the zoo.

Resolution:

• B: No, they weren't (The response negates the previous statement about the children being at home. 'They' is plural, so 'weren't' is the correct past tense negative form of 'to be'.)
• They were (The sentence states where they were. 'They' is plural, so 'were' is the correct past tense form of 'to be'.)

Answer: B: No, they weren't. They were at the zoo.