What causes the shielding effect to remain constant across a period?

This is because the number of orbital shells does not increase. The non-valence orbitals remain the same with maximum electron configuration; with only the valence orbital changing. Therefore the valence orbital is shielded the same, by the non-changing lower orbital levels, from the attraction force of the atomic nucleus.

the correct answer is - d. china has a large and growing population.

the fact that china has a large (the biggest) population, and it is still growing, doesn't hinder the agriculture production in the country. in fact, it can for increased production in the agriculture, since there's more labor force, as well the sheer mass of people produces lots of scientists that come up with new inventions as to how to improve this sector. nice example is the inner mongolia region in china, in general dominated by the gobi desert, but few scientist came up with a nice idea as to how to make the region productive by using plants that are very resistant to droughts, so lots of people have been employed, and the region is becoming greener and greener each year, and the production of food in this region is also increasing.

whats the answer to your question?