21. (1) divergent plate boundaries are those where? a new ocean lithosphere forms b. basaltic magma erupts c. the crust is thinned and extended d. all of the above

This is all you, but i can give you some topics that you could try. if it rained when you want to do something outdoors, it 'limits' that human activity. maybe you want to do an experiment that goes throughout the whole year and you have to have above freezing the entire year. this 'limits' your activity. i thought of those just off of the top of my head. something that gets to me that might be getting to you is you are thinking too hard about what to do. think simply, and if it is too simple, then bump it up just a little bit.

The answer is c, i, iii, and iv. this is because farming and mining deal with harvesting raw materials, construction work falls under manufacturing, and piloting a plane is a service. ii is wrong because making bread is using  raw materials to produce goods, which would fall under the secondary sector, not primary.

As someone who have lived in northern moravia at industrial steel and coal city of ostrava, the closest point is b), but it is questionable. i am not sure how to compare efficiency of largest steel mill in czech republic in comparison to china which is the biggest exporter, but czech steel factories were modernized and run more efficient that 25-35 years ago, and been privatized and owned by mittal. the workforce had been shrinking, so not exactly sure what that c is pointing to (aging of workforce, size of it, or quality). northern moravia is a part of larger coal basin that extend to poland with large coal deposits. this was foundation for the steel industry in the 1800's together with large ore deposits that was mined in proximity of the beskidy mountains less than 25 miles from ostrava. the iron ore mines had been depleted for several decades, and since the late 70's when communists run the country, they were already importing soviet iron ore. coal mining within city of ostrava cased to exist in the 1990's due environmental issues, and also cost. the mines were getting deeper and more costly to operate and there is no more mining within ostrava. couple mines are still in operation in karvina county which is next to ostrava. currently the coal deposits south of the city were not opened due environmental issue which would mean destruction of third largest mountain range in the country. therefore, my conclusion is is b, as the ore and coal resources been shrinking. the biggest challenge not mentioned here is of course competition from russia, ukraine, and china, which can produce steel much cheaper.