Goldenrod gall flies (Eurosta solidaginis) lay their eggs individually in developing shoots of the tall goldenrod plant (Salidago). When it hatches, the fly larva makes a chemical the causes the plant tissue to swell around it, forming a mass of plant tissue called a gall, which serves as a home for the larva.  Downy woodpeckers attack large galls and eat the gall fly larva. Parasitic wasps lay eggs inside small galls and the wasp larvae that hatch from the eggs eat the gall fly larva when it hatches. Only galls of intermediate size remain intact because the wall of the gall is too thick for the ovipositor of a parasitic wasp to penetrate, and the size of the gall and hence the larva within makes it less attractive to woodpeckers than a larger gall containing a larger larva and hence a larger meal. Which graph best depicts the type of selection occurring in this gall fly population?单项选择题