The correct answer is: A. Reduces the strength.
Moisture in concrete can have a number of negative effects on its strength. When concrete is mixed, the water acts as a lubricant, allowing the cement particles to move around and bond together. However, if there is too much water, the cement particles will not be able to bond properly, and the concrete will be weaker.
In addition, moisture can cause the concrete to expand and contract, which can lead to cracking. Cracks in concrete can allow water to penetrate the concrete, which can further weaken the concrete.
Finally, moisture can also cause the concrete to deteriorate over time. This is because the water can react with the cement and other materials in the concrete, causing them to break down.
For all of these reasons, it is important to control the moisture content in concrete. This can be done by using the correct amount of water when mixing the concrete, and by protecting the concrete from moisture after it has been poured.
Option B is incorrect because increasing the moisture content in concrete does not always increase its strength. In fact, as discussed above, too much moisture can actually weaken the concrete.
Option C is incorrect because increasing the moisture content in concrete does change its strength. As discussed above, too much moisture can weaken the concrete, while the correct amount of moisture can help to improve its strength.
Option D is incorrect because not all of the above are true. Option A is true, but Option B is false.