Pertaining to the heart, what substance acts as a lubricant by reducing friction between opposing surfaces?

Pericardial fluid

Pericardial fluid is a serous substance produced by the serous layer of the pericardium into the pericardial cavity. The pericardial fluid reduces friction by lubricating the epicardial surface and allows the two membranes to glide over each other with every heart beat.

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.

b exhibiting favoritism toward the kikuyu people.