1.We heard her … the piano.
a) play
b) to play
c) played
d) had played

2. They watched him … .
a) to approach
b) approach
c) had approached
d) approaches

3. I felt somebody … my shoulder.
a) touch
b) touching
c) touched
d) to touch

4.It is difficult to … the difference between the two cars; they are almost identical.
a) say
b) tell
c) speak
d) look