Nate is driving to see his grandmother. If he drives at a constant speed of 40 \mathrm{~km} / \mathrm{h}, he will arrive 1 hour late. If he drives at a constant speed of 60 \mathrm{~km} / \mathrm{h}, he will arrive 1 hour early. At what constant speed should he drive to arrive just in time?
Answer Choices
A. 56 \mathrm{~km} / \mathrm{h}
B. 80 \mathrm{~km} / \mathrm{h}
C. 54 \mathrm{~km} / \mathrm{h}
D. 48 \mathrm{~km} / \mathrm{h}
E. 58 \mathrm{~km} / \mathrm{h}