In which of these molecules or ions is there only one lone pair of electrons on the central sulfur atom? in which of these molecules or ions is there only one lone pair of electrons on the central sulfur atom? sf4 so42− sof4 sf6 sf2?

The correct answer is SF4.

SF4 or Sulfur tetrafluoride. SF4 is the combination of 34 valence electrons. Around the Sulfur there are 5 electron pairs, in which there are 4 bonding pairs (with 4 flours) and one lone pair (non-bonding pair).

SO42- or sulfate ion have no lone pairs (all are bonding pairs).

SOF4 or Sulfuryl tetrafluoride have no lone pairs (S has 6 valence electrons, 1 for each S-F single bonds and 2 for the S=O double bond).

SF6 or Sulfur Hexafluoride also have no lone pairs (all of the 6 S electrons are bounded to 6 F).

SF2 or sulfur difluoride have four non-bonded electrons (or two lone pairs).

parasites can detrimentally affect their host, but also must not allow their host to die before they can complete their life cycle. some parasites have complex life cycles with many different hosts, and have had to find ways of persisting in many different environments during the change from one host to another. ectoparasites live outside the body of the host, and can more easily  disperse, but are  more exposed to predators, parasites, parasitoids. endoparasites live inside the body of the host, and are therefore protected from the outside environment. but they can be  attacked by the host's immune system. they have therefore developed  mechanisms for dispersal such as  complex life cycles.

the level of progesterone is low at the pre-ovulatory phase of the cycle but it gradually increases and the levels are high at the luteal phase. if pregnancy occurs, the level of progesterone will be high throughout the pregnancy.

if egg is fertilized and the mother gets pregnant, progesterone starts preparing the uterus for the implantation. it starts building up the lining of uterus and make it hostile of the period of pregnancy.