What two concepts does h. g. wells connect in this excerpt from the time machine?

What two concepts does H. G. Wells connect in this excerpt from The Time machine?

At first, proceeding from the problems of our own age, it seemed clear as daylight to me that the gradual widening of the present merely temporary and social difference between the Capitalist and the Labourer, was the key to the whole position. No doubt it will seem grotesque enough to you—and wildly incredible!—and yet even now there are existing circumstances to point that way. There is a tendency to utilize underground space for the less ornamental purposes of civilization; there is the Metropolitan Railway in London, for instance, there are new electric railways, there are subways, there are underground workrooms and restaurants, and they increase and multiply. Evidently, I thought, this tendency had increased till Industry had gradually lost its birthright in the sky. I mean that it had gone deeper and deeper into larger and ever larger underground factories, spending a still-increasing amount of its time therein, till, in the end—! Even now, does not an East-end worker live in such artificial conditions as practically to be cut off from the natural surface of the earth?

the correct answer is: d