Option (A) is correct. According to the textbook context, both statements are true, and the reason correctly explains the assertion. The context clearly states that in hybridoma technology, 'the short-lived antibody secreting normal plasma cell is fused with immortal myeloma cells.' The myeloma cells are described as 'immortal,' and the resulting hybridoma cells 'acquire immortal growth, a typical attribute of a myeloma cell.' The fundamental purpose of this fusion is precisely because B cells have 'a definite life span and die after that,' making them unsuitable for long-term culture. By fusing B cells with immortal myeloma cells, scientists create hybridoma cells that combine the antibody production capability of B cells with the immortal growth property of myeloma cells. Thus, the immortality of myeloma cells is the key reason why they are selected for fusion with B cells in this technology.