Question

$S_1$ : Hungary, with a population of about 10 million, lies betwwen Czechoslovakia to the north and Yugoslavia to the south.
P. Here a great deal of grain is grown.
Q. In recent years, however, progress has been made also in the field of industrialisation.
R. Most of this country consists of an extremly fertile plain, through which the river Danbe flows.
S. In addition to grain, the plain produces potatoes, sugar, wine, and livestock.
$S_6$: The new industries derive mainly from agricultural production.
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Answer 1

R is most logically follows $S_1$ as the 'this' in R corresponds to Hungary. This is followed by P which says that a great deal of grain is grown in the 'extremely fertile plain, through which the river Danube flows' which is introduced by sentence R. P is logically followed by S as this mentions what else is grown in the plain apart from grain. Finally, Q makes a good bridge between S and $S_6$ as it says that Hungary has also made progress in industrialization that $S_6$ further describes.

So the answer id RPSQ.