The following investigation was carried out using flower buds growing on three plants of the same species : Plant 1 The anthers were carefully removed and the buds left open to the air. Plant 2 The anthers were left untouched and a paper bag was tied tightly around each bud. Plant 3 The anthers were carefully removed and a paper bag was tied tightly around each bud. All flowers were opened normally but only those on plant 1 produced seeds. What can be inferred from it?

• A. Only cross-pollination can take place in this plant species.
• B. Only wind-pollination can take place in this plant species.
• C.  Only insect-pollination can take place in this plant species.
• D.  Both self and cross-pollination can take place in this plant species.