11 - C. Does...snow
12 - D. does..own
13 - B. is running
14 - D. is...baking
15 - B. walks - rides

11. ___ it often ____ here in the winter?
 * C. Does... snow
   * We use "does" to form questions in the present simple tense, and "snow" is the correct verb form.
12. How many Barbie dolls ______ your younger sister ______?
 * D. does... own
   * Similar to question 11, we use "does" for questions in the present simple tense, and "own" is the correct verb form.
13. Watch out! Your puppy _______ into the street!
 * A. is running
   * We use the present continuous tense to describe an action happening now.
14. What ______ she _______ in the kitchen right now? - A carrot cake.
 * D. is... baking
   * Again, the present continuous tense is used to describe an ongoing action.
15. Ken usually _______ to school, but today he _______ a bike.
 * C. walks - is riding
   * "Usually" indicates a habitual action, so we use the present simple tense ("walks"). However, "today" suggests an action happening now, so we use the present continuous tense ("is riding").