In cardiac ultrasound, when might an echo-free space be observed both anterior and posterior to the heart?

a) If there is an echo-free space both anterior and posterior to the heart.
b) If there is an echo-free space that is only posterior to the heart.
c) If there is an echo-free space that is only anterior to the heart.
d) If there is an echo-free space not related to the anterior or posterior to the heart.