Osteopenia describes a marked loss of bone density frequently associated with aging. True or False

What was dr. willem johan kolff’s first step in using the scientific process to invent the hemodialysis machine? he built the machine and tried it with various patients, collecting data about its effectiveness. when a patient’s kidneys failed, he wondered if it would be possible to perform kidney functions with an external machine. he researched and collected data about numerous patients who exhibited symptoms of kidney failure. he drew conclusions from studies of various symptoms of kidney failure, and drew a design for the ideal external blood-processing machine.

How can antibodies "remember" a particular bacterial invader or "antigen" on that invader?

Is there any species that went extinct in recent years due to natural causes (not caused by human interaction). if so, what caused it?