Read the following context and choose the best option to answer the question.
Context: I am sure you all own a set ( or more ) of stacking cups , but I just wanted to tell you that this is the toy we ' ve been taking on holidays since my first son was a few months old . They take up so little space which makes them so convenient to pack , and then they double up as bath toys . Kids use them properly ( stacking them ) and improperly ( just banging one against the other ) . We have them with us on holiday now , and they were meant to be for the little one but my son likes to show off in front of his sister and so he is playing with them and building the tower really quickly , which she destroys immediately .
Question: What is the subject of this paragraph ?
Options: 
A. Holiday toys
B. Stacking mugs
C. Playing with toys
D. Double up toys


B