Suppose a concert pianist practises the piano for $6$ hours every day, but to prepare for an upcoming concert, she decides to practise for an extra hour.

What do we know about one more hour of practising $?$

We know that the of one more hour of practising is

A$.$ marginal cost, an hour of reading, missing a visit with her friends, and missing her favourite concert broadcast on the radio

B$.$ total cost; $7$ hours spent practising

C$.$ total cost, an hour of reading, and missing a visit with her friends

D$.$ marginal cost, an hour of reading, if that is her best alternate use of the time