Question

We call these verbs stem-changing because....

Answer 1

1: they have a spelling change in the first part of the verb in all forms except nosotros and vosotros.

Answer 2

1. A (pienso)
2.D(entiende)
3.C(prefieren)
4.A(quieres)
5.C(pensamos)
6.D(cierran)
7.B(cerramos)
8.D(digo)
9.B(dice)
10.D(decimos)
11.B(dicen)
12.C(cierro)
13.B(entienden)
14.B(meriendan)
15.C(quiero)
16.B(dice)

Hope this helps