3.
a. The cat is playing with a mouse.
b. I am building a sandcastle.
c. My brother is diving.
d. We are writing an e-mail.
e. John and Mary are skiing.
f. The dog is sleeping under the bed.
4.
a. They are skating on ice.
b. We are running in the park.
c. Tom and Tina are dancing.
d. I am buying a newspaper.
