Adisease-resistant variety of a sexually reproducing crop was planted on a field with an existing nonresistant variety. after a few years, pests severely attacked the plants. the older, nonresident plants died. however, the recently grown plants survived. why did the new plants survive?

the pests can't attack the new plants.
the new plants had the resistant gene.
the old plants were covered with low-grade insecticides.
the new plant variety was younger than the intital plants.
the recently grown plants had an increased life span.