C. The technology was not advanced enough to explore Mars then, but we have finally reached that advancement.
Explanation:
The technolgoy that the USSR and the USA had back in that time wasn´t advanced enough to take on space exploring, nor even taking good photographs of Mars, now we have advanced technology that has allowed us to explore the solar system with different robots and spaceships that send us signals from what they can observe, we haven´t been able to put a man more far away than the moon.
