Перепишите след. предложения; подчеркните в каждом из них глагол-сказуемое и определите его видо-временную форму и залог.
a) 1. Today scientists are still looking for the substance as a source of energy. 2. The Mendeleyev system has served for almost 100 years as a key to discovering new elements.

б) Synthetic rubber products were developed between 1914 and the 1930s.
2. The intensity of this process is influenced by many factors.