单项选择题

Boarding Concern has been campaigning for several years about the psychosocial problems facing young children caused by early boarding. It has been an uphill struggle, because as George Monbiot points up, the trauma (创伤,外伤) of the privileged has little political purchase. We strongly welcome Monbiot’s call for the issue to get the public policy attention it deserves.
With the publication of Professor Schaverien’s paper in the British Journal of Psychotherapy last year, and subsequent wide media coverage, there is a growing consensus that "boarding school syndrome" is a real consequence for many of those sent away to board at a primary school age. Our members, and others, can give proof to the developmental harm done by premature separation from the security and intimate relationships of family life, which no institution can emulate.
Parents wanting to place a very young child into the institutional care of a boarding school should have to apply to social services and be subject to some kind of process. How we remain blind to this emotional abuse, as George Monbiot points out, is a curious quirk (怪癖) of culture. Perhaps we tolerate it because the schools have become extremely good at advertising themselves in the modern market.
Boarding schools have indeed changed in the past 40 years. Usually, and unless parents live overseas, younger boarders are not being "sent away" but go to a boarding school near their homes because both parents work longer hours than the school day and prefer their children to be with their friends having fun in the evenings rather than being looked after by a childminder or being ferried across town to an extracurricular activity until a parent gets home. Parents appreciate that boarding schools work closely with them to provide security for their children, green spaces, excellent pastoral (田园生活的) care, plenty of contact with home, and the flexibility to allow them to spend as few or as many nights at school as they wish. That is why boarding numbers have risen in recent years. Parents do their best for their children dependent on their circumstances and the needs of their children. They don’t need an inquiry, a committee, a board, an ethics council to reach these decisions. Parental choice matters.
According to the passage, boarding numbers have increased in recent years because ______.