机体对结核菌再感染与初感染不同反应的科赫(Koch)现象，其机制是

A．首次感染结核菌后的反应剧烈
B．结核菌再次侵入人体，结核病灶易播散
C．机体对结核菌已具有免疫力
D．人体对结核菌的自然免疫反应
E．说明机体对结核菌无免疫力

Silicon Valley is a magnet to which numerous talented engineers, scientists and entrepreneurs from overseas flock in search of fame, fast money and to participate in a technological revolution whose impact on mankind will surely surpass the epoch-making European Renaissance and Industrial Revolution of the bygone age.
(46) With the rapid spread of the Internet since the early 1990s, and the relentless technological innovations generated through it, the information era is truly upon us, profoundly influencing and changing not only our lifestyle, but also the way we work, do business, think and communicate with others.
(47) The unprecedented success of the Valley is a testimony to the concerted international endeavors and contributions by people from diverse cultural and racial backgrounds, made possible by the favorable political, economic and intellectual climate prevailing, as well as the farsighted policies of the US government.
Many countries have, or are in the process of creating, their own "Silicon Valley". So far, none has as yet threatened the preeminence of the US prototype. What makes Silicon Valley such a Unique entity There are several crucial factors.
(48) First and foremost, it has the largest concentration of brilliant computer professionals and the best supporting services in the world, and easy access to world-class research institutions, like Stanford University, which continually nurtures would-be that the industry needs in order to move forward. Without these advantages, the Valley would be a different place.
Secondly, it actively encourages, or even exalts, risk taking. Hence, failure holds no terror and there is no shame attached to a failed effort. On the contrary, they will try even harder next time round. Such never-say-die approach is the sine qua non for the ultimate triumph in entrepreneurship and technological breakthrough.
(49) A third decisive factor is the vital role of venture capitalists who willingly support promising start-ups with urgently needed initial capital to get them started. Some would even give failed entrepreneurs a second chance if convinced that a fresh concept might lead to ultimate success.
(50) Of equal importance, bright young people and middle level professionals are keen to work for a new venture at substantially reduced remuneration, as it offers more scope for entrepreneurship and job satisfaction than the established companies. There is also a pride of achievement if their efforts contribute to its fruition.
Intellectual challenges aside, it is a common practice for start-ups to offer generous share options to employees in order to attract the right talent into their folds. This is a powerful incentive to motivate the staff to do their utmost and to share in the company’s prosperity if it reaches its goal Many regard this as the foundation of a successful enterprise.