A balloon is floating in air. Which of the following describes the microscopic cause of the downward force exerted on the air by the balloon?

a) Electrostatic attraction between the nuclei in the molecules in the air and the electrons in the molecules of the balloon
b) Electrostatic repulsion between the nuclei in the molecules in the air and the nuclei in the molecules of the balloon
c) Electrostatic repulsion between the electrons in the molecules in the air and the electrons in the molecules of the balloon
d) Gravitational attraction between the molecules of the air and the molecules of the balloon

You have two photos of a person walking. one shows the person at the corner of third and main streets, the other shows the person at the corner of tenth and main streets. there are lampposts at every corner in this town, and the first picture shows it to be 10: 32: 00 exactly. the second picture shows it to be 10: 49: 30. you know three facts: (1) all of the clocks are synchronized; (2) there are exactly 12 equal-sized blocks per kilometer in this town; and (3) the streets that cross main in this area are numbered consecutively, with no interruptions. what is the person’s average speed in kilometers per hour?