When doing medical research with human subjects, which four limitations are unavoidable? It’s often impossible to repeat trials on the same subjects.
Subjects may report an inaccurate medical history.
It can be difficult to control all possible variables.
It’s impossible to come up with testable scientific questions for human subjects.
There are ethical or privacy concerns to consider.

corona the outer atmosphere of the sun, temp. may reach 2,000,000 c

chromosphere middle layer of the sun's atmosphere

photosphere visible surface of the sun

convection zone layer of the sun which energy travels by convection from the radiative zone to the photosphere

radiative zone layer of the sun which energy is transferred away from the core by radiation

core very dense center of the sun

nuclear fusion process by which tow or more low-mass atomic nuclei fuse to form another, heavier nucleus.

radiation when energy is transferred as electromagnetic waves

convection the process in which heat is transferred by the circulation or movement of matter

differential rotation the way the sun rotates, rotating faster at its equator than at higher latitudes

sunspots dark areas that form on the surface of the sun

solar flare an explosive release of energy that can extend outward

prominences huge loops of cool gas that extend outward from the photosphere

it’s lava came from an explosive eruption