German scientists in charge of the studies ______.
A．thought that the surface soil of Mars is mainly composed of silicon and iron, with a bit of sulfur, chlorine and argon
B．doubted if there is any content of silicon and iron, with a bit more of sulfur, chlorine and argon
C．confirmed that the Martian surface emitted X-rays, which indicated the main composition of the Martian soil
D．indicated that the surface soil of Mars has a main chemical composition of silicon and iron