Please answer this correctly without making mistakes

the answer is

who's...number

who's = who is ( My cousin, Debbie, who's/who is a doctor)

number = how many (the number of twins born)

answer; i believe that the correct answer is (scientific data);

the following key ideas correctly express the key sentence.

restate key idea: most people can learn to swim in ten short lessons.

restate key idea: the majority of water-accident victims require mouth-to-mouth breathing.

restate key idea: an empty, tightly closed gallon jug will support a tired swimmer.  

restate key idea: first aid is to prevent accidents as well as assist in rescue.

explanation: the key idea needs to have the same topic as the key sentence (drowning and safety).